This game is all about vehicles and their craziness. I just feel that it would be really cool if there were some underwater bases that you could get too with a sub. It would open another level of the world that I don't think any other game has ever done.
 
EDIT: Is JC3 going to exist? It looks like it only sold maybe 3 Million units world wide. (I can't find exact figures for this but vgcharts put the xbox and ps3 sales at 1 million a piece.)   http://gamrreview.vgchartz.com/sales/24232/just-cause-2/ (xbox)

Water? I don't like water in my games normally... 
What about a floating city?

You could explore underwater in Lost Odyssey but it was a very limited experience. Subs would be cool in the next Just Cause. I felt like JC2 had the best aircraft and means of air transportation in any game ever (The jet you can buy and parachuting)

Yeah I guess that would be cool, a neat new feature for the sequel, I hope they make one.
 
Also while they are at that why not make it so you can go to space? like seamless transition, maybe a mission to take out a moon base or something, I guess that with the next hardware generation on consoles that can be done, it's an awesome engine they got there.

Underwater bases would fit the spy theme well.  However, I'd want them, and every other location in the game, really, to have a bit more interactivity than the locations of JC2.  If they're just there to be there, then it's a poor concept(Looking at you, Mile High Club).  If they serve a purpose in the game other than housing collectibles, it's an awesome concept.  But I would want them to be polished, with features such as aquatic wildlife swimming about outside the windows, some underwater locales being linked by a concealed tram system of some sort, perhaps?  Really, it's an awesome idea, but it will take quite a bit of work for it to not seem tacked on.
 
Edit:  Explorable shipwrecks, sunken ruins, and giant sea beasts.  I want these in the next Just Cause.

Couldn't you fly the planes underwater in the first one? It was because of a glitch, but I remember seeing a video of that. 

I thought you meant subs as in subtext because from what I've heard the accents in that game are crazy =P

" I thought you meant subs as in subtext because from what I've heard the accents in that game are crazy =P "

There needs to be a Just Cause 3.

" why not make it so you can go to space? "

Just Cause 3 does indeed need subtitles!

" Just Cause 3 does indeed need subtitles! "

Just Cause 3 Just needs to happen. Period. Also, the water was very pretty in JC2, so your idea would be a great addition. 

Well, considering Just Cause 2 went on platinum / classics range, I would assume it did pretty well (probably better than the first). I would sure as heck like to see Just Cause 3.

" @Demmetje said:     

   anyway i hope theres a JC3 ... JC2 is the most underrated game of 2010 "

Everything needs subtitles, man.