I posted this already one year ago, but maybe it is still interesting:
I like the idea of trade-offs. Powerfull (or very cheap) units with some "handicaps".
For example: A concept I namend "anchoring".
- Templars, that can be trained only in cities with a special building, e.g. a cathedral in G&K. Of course, a "standard building" is possible, too. The templar would be a stronger version of the knight (with some extra bonuses against units of other religions maybe),
but could not leave the city radius.
- Militia, that is very cheap to produce and medicore in power, but can not leave city radius neither.
While both ideas where meant as "general" units, they might be thinkabe as UU's, too.
Another idea with some limitations as trade-off for special power:
A
railway gun coming with the railroads technology.
It's movement is (obviously) restricted to railroads. The range will be 4 (or even 5) tiles and it will have more strength than the normal artillery.
Of course, this unit can only be build in cities with railroad connection to the capital (the unit would be too powerful if used as static city defender. Not allowing it to be placed in cities would be possible, too). This artillery will probably be used mainly in defense, but with enough preparation in offense, too.
Many european nations come into mind for this gun.