Translation:Tomorrow will be even better.
I agree; the Chinese sentence implies that today was good. It makes sense to say "Tomorrow will be even better."
I feel that should be the main answer, because geng, to my recollection, implies it's good already.
"Tomorrow will be better" should be accepted as well.
Better entails that it is "more good" than today. Even if today was good, tomorrow will be better.
The dictionary hints on hover say 更好 is "better", but it should be "even better", shouldn't it?
... Will even be better... Is as good... It is not "wrong"... Mind you!