I understand why he was not implemented :
1 character from 2300 A.D.
0 character from 1999 A.D.
3 characters from 1000 A.D.
1 character from 600 A.D.
1 character from 12 000 B.C.
1 character from 65 000 000 B.C.
Present gets a bit too much love, but in a way Crono, Lucca and Marle are the main heroes. Now why would Dark Age get 2 heroes (Magus and Gaspar) ? More importantly, we already have a kickass wizard... What role would be left for Gaspar ?
Personally I would have added a hero from the apocalypse to maximize variety.