Indie developers need an idea for a browser (open standard) game.

Me and a couple of friends from an IT university have decided that we want to create a semi-complex browser game using only open web standards (webGL, html5, css etc). We want to make a 3d game that can run in any webGl enabled browser, and we do have a few ideas about how it should be, but we are stuck at a final design so we're hoping Reddit can help us out.

First we want it to be around collecting / upgrading, to be somewhat addictive and rewarding for the player. That's why one of the idea is to have the game revolve around card collecting (each card is a mob, building, upgrade etc). And based on that, what would go well with collecting is something like an inverse tower defense. Meaning you send out the mobs to attack to towers, or, like in some old Warcraft 3 maps, you have a base, the AI player has a base, and both of you send troops to each other's base trying to destroy it. Other ideas related to this, is to have besides troops and a base, the ability to create building which either give some sort of buff / more money / defensive towers etc.

But we're not entirely happy with this idea, because we don't see much strategy in this, besides having to manage your coin / money to either buy more troops to attack the other base or buying building for buffs etc. We don't want this game to be to easy to beat. Yes we could, and probably will if this is the final idea, make the troops more specialized, meaning some will do more damage, some act as tanks, some add buffs etc. for added strategy / difficulty, but I am not sure if it's enough.

So we are asking the redditors for some opinions : Is it something you will play? / Do you have some suggestions (nothing is set in stone)? Also we don't want to create something to complex, because we don't want to overestimate how hard of a project this will be... for now.