Well I hope to see a Clone Wars survivor reunion. Hera is Cham Syndulla's daughter, so he might pop in, and good 'ol Ahsoka's back... How about Hondo, Bo-Katan, Rex, Echo (still alive), the Bounty Hunters, and of course Ventress? The original rebel, Saw? How about some villains to fill in for the Inquisitor (who's too cool to be dead yet, btw ;))? M-m-m-maul!!... is probably a really long shot. Barriss might make a show, along with some of the Fleet Admirals. Yularen is still with the Empire because he's in the Deathstar meeting scene in ANH (sporting grey hair and moustache). IDK, not as many villains made it out of the CW alive. Beyond all that, I'd like to see if they somehow re-incorporate Starkiller and friends back into the cannon as well.
PS: the saber battle in this episode evoked some of the best elements from TPM and mixed it all kinds a' up with some moves I haven't seen since Katarn was rockin a saber and a blaster in Dark Forces II, but this time, the blaster was also a friggin' lightsaber! Can't decide if Kanan and Ezra vs Inquisitor tops Maul and Savage vs Sidious.
PSS: Sarah Michelle Gellar is supposed to play an upcoming role in season 2, presumably as a pre-existing iconic character. I say she's perfect as a certain bratty teenage princess...