I finished the ER recently, and about half way through, despite never being a fan of JRPGs (or ever playing any, really), I decided to order P3:FES. This might be premature, as I'm sure It while take months for me to finish that game, but where should I go after that? I'd like to play another of the more modern games, like Digital Devil Series or the PS2 Devil Summoner games. How are they? Compared to P4/P3, what's the mix of Story/Combat like?

Those two series would be my main choices right now, but I'm open to others. Or if there are other series which are similar I'd like to check them out.

I've only played P4, P3:FES, SMT: Strange Journey and Devil Survivor (if that last one counts), but I'd recommend Strange Journey if you have a DS. Also, I hear SMT: Nocturne is good. 
 
Catherine is thematically similar to many of those games so it might be something you'd enjoy, despite not being an RPG. 

I'm in the same boat as FluxWaveZ with the additions of P2 for the PSP. I also liked the Strange Journey DS game a lot as well--I hope there will be more.

Non of them is what I would recommend. Zing! No seriously.

Flux pretty much takes it. Especially on Nocturne. I do so enjoy that one.

SMT 3: Nocturne is the best, I feel, Persona 3's good, Digital Devil Saga is fucking crazy as shit and I love it, you really can't go wrong unless you go with P1 because that game is very old

Nocturne is awesome plus you get to team up with dante.

I my self enjoed digi devil saga

If you're willing to look beyond the modern games, I love SMT 1 and 2.  If you've played anything like Etrian Odyssey, you'll sort of have an idea of what to expect.  These games came out on a couple 16-bit platforms, and have been more recently translated into English.  I don't know where GB stands on this sort of stuff, so I'll just say if you want to check it out, use Google.    
The stories of the main line games in general are generally going to be a lot bleaker and apocalyptic/post-apocalyptic than the Persona games (I have yet to play either version of 2, so I can't really speak to that, although I am super excited and looking forward to 2:Punishment).  There is also usually some sort of alignment system in place.  Devil Survivor is a little closer to the SMT games in tone, despite weirdly sharing the prefix of the first Persona game, and it is also a turn based strategy game.

I'm in the middle of devil survivor and so far I'm a fan.

Nocturne features a slightly different version of Persona 3 and 4's combat mechanics, so you could go there. I have yet to finish it, no due in part to the game's rather cruel difficulty. It's good though, even if I still find the demon negotiation system to be terrible. Also, be warned that the game itself is fairly rare, and therefore kind of expensive. I got my copy for around $30, which is more than I'd have to pay for a new copy of Persona 4 at this point.

I've finished all the PS2 SMT games except Devil Summoner 2 at least once, so here's my suggestion.

If you've played P3:FES all the way through you're familar enough with the Skill/Spell names that makes the somewhat less user friendly games, like Nocturne, much more accessible. If you didn't know what "Bufu" was in Nocturne learning it on the MC or selecting it on a Demon in your squad was the only way to look there was no Square Button help text stuff.

Super Famicom fan translated era games...I wouldn't recommend. I've played them a bit. The first person dungeon crawling was disorienting as hell for me, and the legality of playing them is *cough* questionable.

PSX (or PSP remakes) era: The orignal Persona was clunky, even at the time, and the PSP remake looked workable, but still not very good. P2 IS & EP were playable, but I wouldn't recommend them, personally.

PS2 era: With the exception of the Devil Summoner games all the PS2 era SMT games are turn based combat and generally reward striking an enemy weakness with additional attack turns with variations between the titles. They're also mostly dungeon crawlers usually with odd "dungeons."

Nocturne is fairly hard, but if you look up a FAQ to see what Magatama teach which skills (and know what those do already) and have some previous experience with the Fusion system isn't really that difficult. Buffs/debuffs work and you should use them (unlike most RPGs). Being less user friendly than the later PS2 games creates some artificial difficulty in spots.

Digital Devil Saga 1 & 2 are pretty weird even for SMT games, but have unique settings/stories. They run in the same engine as Nocturne, and fixed most of the things in Nocturne that annoyed me. The two games tell a story split into halves, so buy both if you're going to get them. They're worth playing for SMT or RPG fans. Like Nocturne you'll spend most of your time dungeon crawling.

P3:FES has some rough edges compared to P4, so if you like it you'll probably love P4. Personally, I found the story in P4 less "epic", but the characters and setting much more endearing. I played through it twice and watched the Endurance Run, so I've spent an absurd amount of time with it.

Devil Summoner 1 I played through it recently when I was finally able to find a copy. I kind of liked the combat better than the second one. You walk a 3D model Raidou across prerendered backgrounds and get into random encounters. The battles then take place on a separate screen with an "arena" that resembles the area you're in. Even with the reduced encounter rate down status effect active it's still annoyingly high. The story itself advances at a snail's pace and I found myself forgetting just what the hell I was supposed to be doing at times. The story did get pretty awesome by the end, but waiting 40 hours for it was a bit much. The music, while good, there only seemed to be like five tracks and got extremely repetitive. If you're a big action RPG fan it might be worth a look, but I wouldn't really recommend it as a must play.

Devil Summoner 2 I played about ten hours of before playing the first one. They changed up how the combat works a little and you can have a second demon out during battle. I lost interest in it when I first played it. The story suffers from the same glacial pacing as the first one. They apparently have new music in it, but it's still mostly just the first game's repetitive music again. I tried playing it again after I finished the DS1 (there's some minor bonuses for importing your save), but got bored again and dropped it after a few hours. Again, if you're big into Action RPGs have a look, but otherwise I'd skip it.

Nintendo DS stuff: Devil Survivor I played about ten hours of this. I'm admittedly not a huge Tactical RPG fan. I've played through FF: Tactics (PSX) twice and Tactics Advance (GBA) once and nothing else except the first or second Vandal Hearts (PSX) game. The story was really interesting, but I'm not sure if they expect you to grind lots of "Free Battles" or what, but the Mission Battles kicked my ass quite a bit. The battles also took waaaaay too goddamned long and if you couldn't wipe out an enemy party in one round of attacks half the units had a Heal HP ability that made them take forever. I'd have liked to finish it, but it was just soooooo slow.

Strange Journey looks pretty good. I've only played a few minutes of it (I don't have my own DS right now), but I'd like to spend some more time with it.

And this reply ended up being really long.

If you have a PSP, I would really really recommend P1P and P2P. Both are very good, interesting story and I personally find the gameplay more interesting. You don't have social links, but the characters are good and the combat is fun. On the other hand I found P3 battle kinda boring, very repetitive where P1 I actually got to use some strategy.  P4 was a little better but I still would take a fight in the earlier Persona games any day and am sad things like Contracts got taken out. (Having the choice to talk with demons, befriending them and getting Persona's that way? Actually having more then one Persona to work with for each player? Soooo much more depth and less button smash, while still having a auto mode that you can set if you want.)  
 
....Actually I like P1P more as a game then P3 and P4, even if I might have liked the plot a bit more for the later two. Get back to me when I actually finish P2P and I'll see how it ranks. But yeah, enjoyed P1 more as a game.
 
But if those are on the wrong system for you or too old, then Devil Survivor all the way. Harder game then Persona 3 and 4, but I found it a fun challenge. Battles do take some time on some fights, but they keep it interesting so didn't feel like it dragged. 
 
And I would say try out other SMT games, but yeah, a big question is your thoughts on Tactical games. I don't play many, Jeanne D'Arc is the only other one I own, but I do like how it's less a button mash.

If you have a DS, then Devil Survivor is a good spin-off, though the 3DS remake in Devil Survivor Overclocked makes a number of improvements and contains additional content. The DS also has Strange Journey, which is classic SMT through and through. Nocturne and the Digital Devil Saga titles are the best options on the PS2 (next to the Persona games), and the Devil Summoner games are entertaining, as well.

@VikingRk: I know you already finished the ER, but playing Persona 4 is still a blast so I would suggest you go through it at some point. I would also caution you, as someone who is currently working his way through P3: FES, its more obtuse then 4 and I would suggest that you be ready and willing to look for help when you need it.

Nocturne is a must. I also loved Digital Devil Saga and at least enjoyed Devil Survivor on the DS.

@evilpaul:

Just out of curiosity, what was the difference for you in enjoying Strange Journey and disliking the first SMT games? Is the field of view wider, or does it auto map without the use of a spell or something? I have not had any experience with it, as I was kind of turned off by the lack of any post-apocalyptic urban setting.

@Commisar123: More obtuse in what way? I'm around 15 hours in nearing the end of may, and things don't seem to complicated at this point.

@VikingRk: I don't think they explain the systems in place as well as they do in Persona 4, but since you watched all of the ER, you probably had no trouble understanding the basic stuff. Its pretty soon after the part you are on that things get a lot more difficult though so I wish you the best of luck :)

Definitely Nocturne. JRPGs don't get much deeper in terms of gameplay than Nocturne, even to this day.

Etrian Odyssey 3, I'm in love with this game right now. It's not a SMT game but it might as well be.

If that's too hard however, I highly recommend Strange Journey.

DDS1+2 and Nocturne also, some of my favourite PS2 games.