This might be venturing more into Deviant Art land, but considering we have a new generation coming up, it's always possible the old trifecta of Unicorn/Earth pony/Pegasus might be changed up a bit if Hasbro wants to venture into some of the weirder ideas they might have. Imagine a pony where the entire formula was thrown out the window and we got some discord-level chaos world originality going on.
We've had a few offshoots over the years, with breezies, bats, and now the kirin. The fan side of things has taken the concept far beyond what the show has conjured up though.
I'll slap a few below, but feel free to dig around or bring up some that you've run into over the years!
Witchfae
These guys have been relatively popular over the more recent years, usually magic focused. There is an entire DA page dedicated to them.
Pollen Ponies
Half flower, half pony. Not nearly as common as the Witchfae
Pond Ponies
Another species with a DA page. They expand far beyond just ponds, with some like the one above rockin' saltwater.
Succubus Ponies
Does ponyland need some demons? We've already covered pretty much every other fantasy trope.
I'll let you bring up any others! There are a crazy amount of them. Discuss below!