+1.making every civ "viable everywhere" isn't possible without big changes (the tech trees become way too different in the late-game)
but giving all the civs a viable way to use player skill to pull ahead during the first 25-30 minutes of a game is realistic.
Any early eco/military bonus (I consider early all that applies to Dark, Feudal and early Castle) should mean a heavy advantage at max, not a game lost in advance.
Eg. today Mesos vs Koreans on land dominant or Viking vs Franks on water dominant aren't even worth playing. For two "expert" players, chances for the one who got Kor/Fr in these matchups should go from ~3% to 5-15%.