Holy crap. Just watched the trailer.

Kanan looks cool (Sabine painted some Mandalorian Jaig eyes on his mask!)
Freaking WEDGE defecting (with Sabine, according to Tobar)
Y-Wings (Tobar says that the whole base and everything is taken from a West End Games scenario)
Battle Droids! Neat!
A YT-2400!
More Mandalorians. It’s cool because they were originally Imperial Supercommandos, which is what these guys are. Very exciting!
Dark. Freaking. Troopers. If they have the Arc Hammer I will lose my mind. If Kyle Katarn shows up, I’m sold forever.
Tarkin’s back!
Cham’s back!
Maul’s back!
TIE Interceptors (with red stripes)!
Nightsister magic?
The Bendu (Bindu) looks amazing. Definitely going to be seeing some really trippy Mortis/Korriban level stuff in this series.
Sabine gets the darksaber!
Scout Troopers!
More Inquisitors? Sith spirits? HMM!

And of course, Thrawn.

I’m a bit in the minority with him. By the end of the Thrawn trilogy, I hated the character because he’s just too perfect and infallible and it gets really stupid. Hopefully Rebels will turn him into a decent character, though. I’m more interested by the precedent it sets: if Thrawn can be recanonized, who’s next? We could get Mara Jade or who knows. That’s what’s really exciting to me.