hmmm
at the moment doesn't it just halv the chance of going up a level every level, couldn't you just continue that trend so after the 1/32 chance to produce a great leader, 1/64 to get heroic, then 1/128 to get legendary then 1/256... I still like the way it goes in smac(even now i still play more smac than civ III) where each gives you a 12.5% combat bonus, with elites getting a +50%!
anyway one bit i really liked too was in some of the command and conquer games the units gaining abilities as they went up levels, like natural healing or the ability to attack ait units (again can you tell how much I play smac the game would have been completly different if I had some way to get my hands on just one unit with three special ability slots)...
what i thought you were going to talk about, from the thread title was unique-unique-units units that you only get to build one of...
fun ideas..