I thought I'd mention a couple of (free) games which might be a source of ideas for clash.
People's Tactics (http://www.peoplestactics.com/) is a great hex-turn-based-strategy game with a lot of combat detail. One of the best features of it that could be used by clash is the way air units work. I would advise playing it, but I'll try a brief description. Basically, instead of pushing air units around the map, you click attack and it lights up all the squares in range. You click attack and bingo the attack is done. What I like about this is that you can never loose an air unit because it runs out of fuel like in the civ games. One modification I would suggest to perfect this would be to select a landing square as well as an attack square so the unit doesn't just return to the starting square. This would also increase the range if the landing square is nearer the target or reduce it if it is further away.
Xconq (http://www.xconq.org/) is another cool hex-turn-based-strategy game. It is incredibly versatile and fun to play. Although this has got lots of features, one thing I noticed was having sides not named after sides in the real world. I think the use of made up words could be of use to clash for random games. This would avoid confusion of related ethics groups, religions, civilisations etc. Perhaps some kind of simple random word generator could be used?
If anyone else has found other (free) games of interest and inspiration add to this thread.
Civ 3's air combat works much like that of People's Tactics, though they don't make use of a landing square, which is a cool idea for that system.