If you do add sound, for the love of god have a sound volume slider, or let us turn it off. I hate having to turn my speakers down or fiddle with the volume mixer every time I play.
Okay, a few things I can think of that i think would be good. not sure if all of them fit the game style you want to make, though.
Have you thought of perhaps having particle effects of some sort when the cannon shoots? Perhaps even have the rounds explode upon impact? (I assume these will be cartoony tanks with cartoony physics and such). Maybe even have tank treads making small puffs of dust or gravel when it drives over certain things. I don't know if you have "outdoor" environments planned yet.
Infantry. In team-based or co-op games I think there should be infantry of some sort controlled by AI. You would probably need to have a coaxial machinegun on the tanks though to deal with them. They wouldn't do too much damage unless they had AT grenades and rockets, but there would be a lot of them. Fun when your cannon rounds explode a whole bunch of them.
Tank reload. I think that there should be the main canon that has a small cooldown on it, and a machinegun that shoots faster but doesn't do as much damage, and has a cooldown timer after a certain number of shots are fired. Perhaps the machinegun bullets ricochet off walls easier than normal tank rounds.
Customizable tanks. I'm thinking this is what someone else was talking abooot. You could have several parts to choose from... different treads, chassis, turrets, barrels, engine... and combining them in different ways gives you tanks that operate differently from each other. Put wheels instead of treads on and you can go much faster over smooth surfaces but suffer on the rougher ones. Give yourself a more powerful engine that is more likely to leave you immobile for a short time (or something) when hit with a tank shell. Of course, in non-team games I'll want to pick what color I want my tank too =p
Crosshairs. I think that there should be a crosshair of some kind so aiming at more distant tanks is a bit easier. Of course, you could leave one out and have it just be part of the gameplay style.
Large map. I think that most maps should take up more than 1 screen, and have the screen scroll as you move around. Might be hard though, I've no idea.
Landscape/obstacles. There could be trenches, for example, where you move slower but you take less damage while inside them. Water, where you just move slower in general (and with deeper water you'd need a special engine, perhaps)
Different tank barrels shoot different rounds. Perhaps some rounds bounce off of walls easier, or some rounds are smaller and less damaging but shoot faster. Perhaps some can be lobbed over certain obstacles. Replace a barrel with a plow of some sort and ram tanks to do damage and clear certain obstacles/mines.
I dunno, most of this stuff doesn't imply 'toy' tanks (except maybe the interchangable parts) but that's just what I would like to see.
EDIT: Perhaps make the collision boxes on the tanks slightly smaller so that it's harder for them to be caught on things, and maybe even make the tanks longer than they are wide? (depending on chasis maybe?)