New races are quite likely to someday come to a crown store near you, similar to how we have Imperials now. I could easily see Maormer, Goblins, Reachmen, Akaviri... since those would be not all that much for them to do, all these races and their "racial crafting styles" are already in the game, and the lore has no reason against them being added to Molag Bals victims... and becoming heroes after their escape from coldharbour... they just need to slap together some racial passives and bundle them for players to purchase...
I could even see some of those races as "allaince specific unlock" - like, say Maromer can be taken for pact and coveneant, but not dominion because of the "ancestral enemy" thing... or Akaviri for covenant and dominion, but not pact because the memories of the last invasion ten years before ESO is still too fresh... or Reachmen for pact and dominion, but not covenant due to them being the bad guys in some of the current storylines...

More exotic races like Imga that have curiously been absent from ESO, or are sort of recently gone extinct like Kothringi and Lilmothiit may also be possible, with an "last survivor" theme... (sure, the knarhaten flu wiped them out... but... if a player character was in a coldharbour prison cell while that happened, they might still be around, right? The specias is doomed regardless, since "last survivirs" cannot really repopulate due to genetic diversity ssues, but the powers that be -could- allow such as player characters...) though I would hope they would be sold at artificially inflated prices to really keep them "rare". Say, at least double what any "common" race might cost to unlock.

Races that are long gone like chimer, snow elves or dwemer on the other hand would be very unlikely due to lore reasons. As would races that make no sense in the story, like dremora and their ilk. For that matter, so would races that create other problems, like, say, minotaurs or ogres, for being oversized and clipping through doorways and such... Personally I hope we never see those, not like there aren't enough nifty other races around...


New classes... eh. My opinion towards that is rhe same it has always been... I really dislike the idea, both on account of already having 12 characters, and even moreso because of the "waaah, why wasn't this class available back when I made that character, it would have been sooo much better for the backstory..."
Personally I really hope that any "new class" may come in ways that can be refitted to an existing character - like I outlined in my old class morph idea: https://forums.elderscrollsonline.com/en/discussion/240186/class-morph-idea/p1
Of course, IF they add a new class, they could lessen those issues with also adding class change tokens... but they have stated that is something they do not plan on adding, so...

Also, there is more to character then classes. Another possibility is them adding the warden class piece by piece, each skill line with a new guild... frost mage guild, animal trainer guild, nature mage guild... just spin an DLC questline around it, and they can do that just like TG and DB. And of course, once they start... they could keep adding guilds... https://forums.elderscrollsonline.com/en/discussion/306759/additional-guild-ideas