If Regina never casted the curse and was banished I assume:
-Cora and Regina teamed up and took over other kingdoms. Cora taught Regina about patience. Regina would have been broken that she never got to cast her curse. Zeleena more than likely would have confronted them and then teamed up with them and took over Oz.
I assume they would be 3 of the most powerful Dark Magic userers to ever live. I am pretty sure Cora would have wanted 1 of those necklaces that Zeleena had to increase her magic. With Rumple gone their would have been no one to challenge the Mills.
-August would have taken over his father's craft as shown on screen.
Also Wonderland would have remained the exact same. Nothing would have changed. The curse had no effect on the events in Neverland. Rather it did or didn't happen nothing would have changed.
I wouldn't mind seeing something of a side story on it.