Oh oh oh, welcome friend, let's band together on a project that'll suck the few minutes of free time we managed to save from our bosses, wives and the rest :)
We should defintetely consider what to do exactly, and in which order. For instance I just thought it to be nice if the monsters weren't static, but could move a bit around their "origin", flee when wounded, etc. Wow, so much to do...
For the combat system, AD&D can be good enough (there's a version of the rules on the TSR site that can be freely implemented).
Perhaps a fully object game system would be better, because it would be easier to extend the game (for instance to get it multiplayer or put a GUI on it). I think something like a quick turn-by-turn system (with a 3 seconds turn for instance) would be nice, because it will allow a very interactive gaming without the hurdle of a true real-time system (hard, especially for combats).
OK, ok, I'll stop that now : I'm right now working on the "world generator" first, When I'll have something ready I'll post a brand new Code Catacomb node!
Hey, can't this be useful? battle.pl

update: and I didn't even talk about magic! I had this idea (let me know what you think of it) : it would be nice to be able to find scrolls (perhaps by killing some monsters, or as special rewards); then you could either use the scroll (for instance "fireball") once, and it'll be burnt out, destroyed, reduced to ashes; or you'd be able to LEARN the spell from the scroll (takes time, energy, mana, whatever) and then be able to use it as you like (but of course you then will need enough mana, etc). Wadyathink? I LIKE this idea.