After reading some of these, people seem to think that Johto has used up a lot of their pokemon for Alolans. But after I went through the list, there is actually a ton of pokemon left they could use. I am not going to list starters for obvious reasons. Also not going to list baby pokemon, but maybe the last stage of them. Also not going to include Unknown or Legendary Pokemon in this.
Sentret and Furret
Hoothoot and Noctowl
Natu and Xatu
Mareep, Flaffy, and Ampharos
Bellossom
Hoppip, Skiploom, and Jumpluff
Sudowoodo
Aipom and Ambipom
Sunkern and Sunflora
Yanma and Yanmega
Wooper and Quagsire
Wobbuffet
Girafarig
Pineco and Forretress
Dunsparce
Gligor and Gliscor
Qwilfish
Shuckle
Heracross
Remoraid and Octillery
Teddiursa and Ursaring
Slugma and Magcargo
Mantine
Phanpy and Donphan
Stantler
Hitmontop
None of these pokemon make any type of appearance. Almsot every single Johto pokemon made no appearance. We know Pokemon has been using Pokemon Go as a promotion tool for TCG. It would be the perfect time to give Johto pokemon some new looks. Even though a lot of them have evolutions in Sinnoh, I don't think we should count them out just because of that. Megas I think are ones we need to count out though.
Also, I saw a suggestion by someone on here and I saw a leak, but the leak was probably fake, but a Ice Type Stantler would be amazing. Especially when it has been dubbed Santa's Reindeer in the anime.
Most of all Johto pokemon would be perfect for Alola.