The Platinum dex of 210 will be used as our baseline with various additions and also some removals, their rationale being explained when necessary as I go along.
Can't exist yet (-8):
Sinnoh fossils (Timeframe for being alive normally was millions of years ago and there's likely no fossil revival tech)
Rotom (No appliances aside, DPPt made it clear Rotom was a recent discovery in-universe so even just having the base form is a no-go)
Porygon family (Self-explanatory)
Sinnoh Pokemon/relatives that just weren't formally listed (+10)
Phione, Heatran, Regirock, Regice, Registeel, Regigigas, Cresselia, Darkrai, Shaymin, Arceus
Newly added pre-existing Pokemon/lines with no Hisuian forms (+16)
Gen 1: Vulpix, Ninetales
Gen 2: Qwilfish, Teddiursa, Ursaring, Stantler
Gen 3: Spheal, Sealeo, Walrein
Gen 5: Basculin, Thundurus, Tornadus, Landorus
Gen 6: Sylveon (if new family extensions outright are being added I don't see why Sylveon would be off-limits)
Gen 8: Regieleki, Regidrago (see above)
Newly added pre-existing Pokemon/lines with Hisuian forms (+22)
Gen 1: Growlithe, Arcanine, Voltorb, Electrode
Gen 2: Cyndaquil, Quilava, Typhlosion
Gen 5: Oshawott, Dewott, Samurott, Zorua, Zoroark, Rufflet, Braviary
Gen 6: Goomy, Sliggoo, Goodra, Bergmite, Avalugg
Gen 7: Rowlet, Dartrix, Decidueye
New Pokemon and Evolutions (+6)
Kleavor, Qwilfish evo, Bear Pokemon evo, Wyrdeer, Basculegion, 4th Genie
Current Tally: 256 Pokemon