I've been trying to cut down on it to try to avoid being too pessimistic and save myself the hassle, but as of now there are seven still MIA that are kinda non-negotiable for me.
Marowak (Kanto), Ursaring, Shiftry, Castform, Sawsbuck, Klefki, Toucannon. I'd say others like Dhelmise and Volcanion, but they're not as non-negotiable given their almost certain absence compared to how much I like them.
Hm I don't think this list is too unlikely.
Marowack I'd still has a decent shot if it was remembered enough to get an Alolan form. Plus there's sort of nostalgia towards it due to the whole Lavender Town thing in gen 1.
Ursuaring I personally think stands good grounds to get in as a galarian form, but normal wouldn't surprise me.
Castform i don't think has ever been in the wild so I don't know if I see Gamefreak just handing one out.
Swasbuck feels pretty likely, if not the likeliest. The wild Area Cycles through weather, so you could already have spring or fall appear in the rain, the winter one during the snow, then you could have thee other two appear wherever.
Klefiki I'd be 50/50 on. It gets decent competitive use and Gamefreak seem to love their object Pokemon. If vanish and it's evolutions can get in, nothing really stops Klefki.
Toucannon. Would be cool, but has a lot of competition between other popular bird pokemon. Gamefreak might not want to add to many of one type so they can balance the board across all 18 types.
Dhelmise is a klefki scenario imo. It sort of exists but its unique enough to earn a spot imo. Overall I'm rooting for them for you, good luck!
Also apparently Mew is in this Dex now