I also vote for the SS 50 --> 100. It is a major change allowing for balancing out models and justifies a new edition, but it doesn't change the rules, which are mostly working fine.
Then some revision of wordings and models, refreshing them and cleaning up the whole errata/0 upgrade fixes.
I could also imagine some extra level added to the game, but who knows what that could be?
But I guess for the time being, we will see a couple of alternative models first. Book 6 could be made up of story encounters with a few models thrown in (or not even); maybe some solo-missions. that would actually be very cool.