Re: rainboom-less AU
I can see Pinkie being a more mellow version of herself. Whatever that means.
Applejack probably wouldn't have been much different. She was already homesick even before the rainboom, and it was only a matter of time before she came home again.
Twilight might have actually made more friends. Maybe. I mean, I don't think she'd have had as much of an opportunity to shut herself in a library all day with her only meaningful social interaction being a baby dragon and a princess, and occasionally her family.
Fluttershy had pretty much already discovered her affinity to animals before the rainboom, so I don't imagine there'd be much of a change either.
Rarity might end up being a late bloomer, and may or may not eventually discover a fondness for gems. But I imagine she'd be an aspiring fashion designer. Just with less sparklies in her designs.
Rainbow Dash's case is a bit more unpredictable. It would depend on
why the rainboom didn't happen. Not enough speed, which lost her the race in the end? Or was the accidental rainboom just a one-in-a-million fluke that simply ended up not happening?