God Harry, What Have You Done? 
 

I really enjoy the over-poweredness of the spas. I'd say using secondaries would be a good way to try something new.

The challenges are there for a reason.  Go through all of them and try and get some of the ones you don't have.  Or use guns that you are absolutely terrible with to try and get better with them.  I know I always get pissed off when I see someone owning with a gun that I know is absolute crap. 

SPAS with a grip does pretty good even on places like Sub Base or Skidrow.

I'd also try using the MP5-K with the USP & Tac Knife. Very good combo with Scavenger, Lightweight, and Commando.

Have you mastered every weapon? If no, then it should be obvious. Master the weapons you haven't mastered and make sure you try every weapon. You shouldn't have to ask us, just try all the different combos of perks and equipment and upgrades and even game modes.
 
EDIT: My favorite weapon is the AUG-HBAR Foregrip with Scavenger, Stopping Power and Ninja. I got most of my nukes with it. I also dig the M240, SPAS-12, AK47 and FAMAS.

RPD is madness.

Y'know, when I first used an ACOG'd Intervention, I laughed at myself. Never thought I'd be any good with it. Grabbed the revolver for my secondary, thought the same thing.
 
Then I put Stopping Power on it, and got used to the ACOG sight, and fell in love with the beast. Shot to the upper chest/face is pretty much an insta-kill, works at Long/Medium range and doesn't do that bad at close. As soon as I got the hang of the Revolver, that turned into a close-range murder machine. I got my first like, 15 kill-streak with that setup. Airstrike, Harrier Strike, Stealth Bomber, then a bunch more kills. As long as you get the hang of the ACOG scope, you can hold down an area pretty well (Afghan I think, with the downed C-130, back-side of the pillboxes/bunkers, can usually rack up kills from there as people try to run through the bunkers). Love having Tactical Insertion too, makes that class so much more fun.
 
Though anything with the Thumper is usually great too. Funny how mad people can get.
 
I also have a "sneaky" class, which is a silenced RPD with a Stinger AA Missile, Cold Blooded, Ninja and Slight Of Hand. I once hid in the middle of an enemy team before bolting from my hiding place and murdering like, six dudes.

ACR with the scope is fucking INSANE!

Try using the FAL. Once you get used to it it's pretty funzies.

i was getting bored before.. but then i decided to try and get all the emblems for the perks. what a challenge. just got lightweight and sitrep left.