That's what we want but seeing as Junichi Masuda is returning to direct and he wasn't involved with Pokémon Platinum, I'm genuinely concerned.
I can understand that, but I'm pretty sure we never had such a disastrous Pokédex since DP. Yes, maybe Gen6 and Gen7 fell short in terms of type diversity of the new Pokemon (XY had 1 Bug-type and 1 Poison-type family) - but those dexes made up for that. So that's why I'm pretty sure they learned from their mistakes.
I think that the Regional Dex will be Platinum's with the addition of all the Sinnoh Legendaries not in the Platinum dex, all the Regis, all the formerly dual-slot obtainable Pokemon, Sylveon, Mr. Rime, and Ditto because it's needed in order to get Phione.
Why should MrRime be in Sinnoh when there's zero reason for Galarian Mr.Mime to be found in Sinnoh?!?
Which Sinnoh Legendaries were not in the Sinnoh Regional Dex?!? Do you mean the mythicals?
Dual slot-pokemon: sure.
Sylveon: sure.
Ditto: no doubt.
MrRime: not likely.
I think a few post Gen IV Pokémon will be in an updated Sinnoh Dex with a focus on the Fire and Ghost typing plus a couple Ghost, Fairy, And Dark Pokemon just to add more variety. The Litwick and Darumaka lines seem to be the best choices at new Fire types
Litwick could be fun in the Eterna Forest Mansion. But where would you put a flaming doll monkey/gorilla?
I could see Heatmor in the Great Marsh, Fletchinder on the cliff-route up to Celestic Town. Salandit, Turtonator and Volcarona at Mt.Stark. But in fairness, I don't see them adding that many Fire-types.. if any at all.
If they have Platinum's Dex, we have at least Infernape, Rapidash, Flareon, Houndoom, Magmortar and Rotom-heat.
In terms of Fairy-types, you've got a plethora of options: Togekiss, Sylveon, Azumarill, Mr.Mime, Gardevoir, Clefable and potentially Mega Altaria.