I added the 10 new evolutions, then reordered the hoenndex so its more grouped into areas (rather than how you personally meet the pokemon), and cut it up into those.
Looking at it like that, the more boring areas seemed to be the routes around Fortree, the safari-zone, Shoal cave, and Seafloor cavern.
I carefully added pokemon from gens I,II,IV,V to each group, trying not to overdo it, but adding at least something interesting in most of them.
This is the result:
-Poliwags in freshwater places, as surfing on those is REALLY boring (actually, it could be Tympole though to avoid too many Water Stone evolutions, but Water/Ground is overdone in comparison to Poliwrath)
-Pinecos and Cherubis I was thinking to add to petalburg woods at first, but now I've remembered the berry-tree encounters in XY, and seeing these two both imitating stuff that falls from trees, if that feature is in, those two would be cool. (Forretress adds a Bug and Steel diversity that isn't Escavalier/Scizor for a change, while Cherubi is unavailable in XY)
-Buneary above Rustburo somewhere (Hoenn lacks friendship evolutions and Buneary was one of those actually unavailable pokemon in XY).
-Drifloon (an early ghost-type is so refreshing) perhaps doing it's weekly appearance on the shore north of Rustburo somewhere
-Frillish around the Abandoned ship. Sold.
-Alomomola and Mantine to add diversity to the open sea surfing experience
-Shellos simply deserves a place in this region somewhere for almost making it into RSE, although it's ending type combo is not that exciting with Swampert/Whiscash around. It even has a land vs sea vibe in it's form difference, Im convinced that was a thought somewhere in its origin.
-Klink is really the only proper candidate to add to New Mauville (Tynamo doesn't strike me as something that would live in manmade places), probably even makes more sense than a natural cave for it like we've seen so far.
-Cottonee and Hoppip, some floaty Grass types east and west of Mauville (also a second Sun Stone user and an extra Fairy-line in this Dark dex)
-Darumaka for the Fiery path and around the volcano, brofisting with Machops ;D
-Clefairy (I was surprised they put Jigglypuff in the games that promoted the Fairy type instead of Clefairy.) and Elygems are highly appropriate for Meteor falls what with Solrock/Lunatones around the place, especially if they get any sort of expansion or secret area /wishful thinking
-improving the desert with some Sandstream (Hippopotas. which actually makes freakin sense compared to them on the cliff on the Rhyhorn path in XY.. ), gliding scorpions (Gligar) and more flora (Maractus).
-Finneon for more freshwater fishing diversity. (yet another of those few unavailable in XY)
-Yanma, Carnivine to make the paths around Fortree more exotic, perhaps some fake trees to go along with invisible chameleons.
-Larvesta for that lone cave east of Fortree.
-Misdreavus as a third ghost in the tower (it has like 5 stories IIIRC lol)
-Litwick fits Mt.Pyres theme very well, and Bronzor adds the same ritualistic feel Chimecho is there for already.
Safari:
much could be done here, but it's kind of unneeded with how diverse and colorful Hoenn already is.
-Mankey (more fighting diversity always welcome)
-Snubbull (as is Fairy)
-Stantler (Girafarig is in there so wynaut)
-Teddiursa (Phanpy is in there so wynaut)
-Nidorans for some serious 3 stage poison line action (and as a nod to Kantos safari)
-Zebstrika (it's just cool and a zebra xD)
(there'd still be room for some Hoothoots or Shuckles or Houndours (obvious Emerald bias I know) or more... like Mareeps...but we've had Mega fabulous-line in XY and BW2, so rather not)
-Shellder and Qwilfish for more spiky underwater stuff
-Tynamo and Dwebble for underwater caves (seafloor cavern)
-Seel and Sneasel for Shoal cave and Ice diversity
and that brings the number quite close to 300 already. I've had more thoughts behind thee additions than I bothered writing by the end of the list..
Last few is either some fanservice I'm hesitant to add (more pseudo legends, eeveelutions...), another starter set gaining new megas (Johto if any), or who knows what.