Johto is not necessary, with a little imagination it is more than necessary. With the map of this new region you can implement the pink pokemon, alola - galar forms and missing evolutions of existing pokes (they wanted new pokemon because there they have it). Sylveon normal type, ok no.
New gym and farming routes (NPC and objects), hidden dungeons with Mythical Pokémon (the idea is to discover them or make a kind of puzzle, etc).
New hordes for shiny hunters with probably hordes of dratini - staryu - shellder - horsea that are no longer rare or semi in my opinion.
Mini pokemon games available to hang out, for streamers content or some team event (guilds) etc and many more things that occur to me but you already understand the idea, many things can be implemented with the incorporation of Johto but it depends on the administrators how and when they will do it and what new resources they incorporate to keep the community active.
To forget me new vanitys referring to Johto's initials, legendaries, etc. Ideas flow but reality is different for now.
