Michelangelo
Prince
So you are suggesting to broaden the gap between units, so a tank would be much stronger compared to a spear. But than you again fill it up with intermediate units, so the power-difference is the same againI think that there simply must be more difference between unit's strengths, and maybe more intermediate units.
Somewhere in this reasoning you have to make up your mind what you want
I think the power distribution is about right. In CIV4 and C4W the cavalry was a bit overpowered but that has been fixed with a intermediate unit (cuirassier). If there are to big power jumps, the first to get there will win, always. That would make for the single path to victory problem and would kill the fun of Civ. The way it is now there is still an advantage to have a stronger unit, but all is not lost (except if you happen to be an AI )