Hmm. By the time we *can* build another moon, we probably won't actually *need* to because we'll have advanced so much.
I'd say keep things simple - say from present day (+20 years or so) to 2300. If we do it in terms of months (can we actually *have* months in this game?) that would give us between 280-ish and 3,200-ish turns to work with. We don't want it to last *too* long, do we?
I think things tend to get a bit bland the further on in the future you go.
In terms of combat - however you like it. Projectile weapons will be good, and as for beam weapons - well, one could use lasers to burn holes in armour/spacesuits. The goal in combat, though, would be either to disable or to destroy the enemy, and the customary way of doing that is to cause explosive decompression in whatever conveyance the enemy is travelling in.
Later on we could have computer virus weapons that can force units to skip turns.
Artillery might be useful after all, you know - what about self-propelling shells?
Also, once people start having the 'independence' civic we could have massive waves of 'barbarian' units - Earthbound nations trying to reclaim the colonies.
Does anyone know if it is possible to program in Civ II-style missile weapons? That'd be cool.
EDIT 1:
How about the first few units have no combat but other special abilities? That could result in a really diverse 'army' by the time the first brute-force unit comes along.
EDIT 2:
What do you guys think of the map, btw? Is it usable?