Alright, so yesterday I made a list of the potential candidates of Johto Pokemon that could get Alolan forms. I excluded the ones that got cross-gen evos in Gen 4 and Mega Evolutions.
Pokemon that weren't in Sun/Moon - Sentret line, Hoothoot line, Natu line, Bellossom, Marill/Azumarill, Hoppip line, Sunkern line, Wooper line, Unown*, Wobbuffet, Girafarig, Pineco line, Dunsparce, Qwilfish, Shuckle, Teddiursa line, Slugma line, Remoraid line, Mantine, Kingdra, Phanpy line, Stantler, Tyrogue/Hitmontop, Smoochum, Raikou*, Entei*, Suicune*, Lugia*, Ho-Oh*, & Celebi*
Pokemon that were in Sun/Moon, but can get one since it'd be a different universe - Chikorita line*, Cyndaquil line*, Totodile line*, Ledyba line, Spinarak line, Crobat, Chinchou line, Cleffa, Igglybuff, Sudowoodo, Politoed, Slowking, Snubbull line, Corsola, Delibird, Skarmory, Smeargle, Miltank, & Blissey
* = I doubt the starters, Unown, and Legends would actually get an Alolan form, but who knows