I don't understand. If the three best characters are likely to return, why should we expect disappointment?
On the Cassandra vs Rock debate... It might be worth thinking about roster diversity in terms of fighting game archetypes. The game has plenty of cute girls, sexy ladies, sexy guys, monsters and freaks. But Astaroth is the only "big guy" on the roster. And even then, he's not a true grappler. Sure, Cassandra is more popular, but ANY character with boobs would be more popular than Rock. Boobs > pecs. It's just science.
But if you want to appeal to new players, who may have experience with Zangief in Street Fighter or King in Tekken, it's really useful to have a character they can comfortably transition into.