I just got simcity3k, haven't played one of these since sc2k, but before that I modded civ3. I was looking to create a more engaging combat system, and more realistic.
I made normal units have 4hp, veteran, 6 and elite 10hp. I gave pretty much every unit double movement, even workers. I changed some upgrade options, like ironclad to submarine, warrior to swordman to marine. I reduced the food needed to support a pop to 1 per (hard to abandon a city now ;P)
I gave bombard ability to subs and tanks. Seems odd to me how they dealt with subs, the whole point of the submarine is that its hidden and you can torpedo ships and sink them. So in Civ3 when you attack with a sub, it *comes to the surface* and shoots torpedos in plain sight! How ********. And of course now that it is on the surface a galleon can defend and defeat it, HA HA!
Anyway its been fun so far, pretty dangerous on diety with the higer movements. The free units the AI gets when settling can be on your ass really quick.
Opps forgot...
I don't know how they dealt with retreating fast units but it was obviously based on 3,4 or 5 hp since no veteran or elite unit will retreat now.