When it comes to Legendaries from former regions/Gens, the ones that would fit the Galar format for me are as follows & my reasons behind my picks:
1. The Kanto Legendary Birds, because they have been linked to the seasonal powers of Summer (Fire), Spring (Lightning) and Winter (Ice).
2. The Johto Legendary Hounds/Dogs. Aside from being similar to the Kanto Birds, they match pretty close to the signature Legendaries of Sword & Shield.
3. Lugia & Ho-Oh. Being the Masters of the Birds and Dogs basically means they go where there kin are. Not to mention Ho-Oh is basically a phoneix and rebirth plays a large part in Celtic lore and Lugia's ties to real world Celtic sea/water creature myths-legends.
4. Mewtwo. According to the site Serebii, Mew will be there. Since Mew and Mewtwo are basically 2 sides of the same coin, wouldn't be right to have one and not the other.
5. Celebi. Dubbed "Guardian of the Forest" and it's time travel ability, it would hit in so many ways.
6. The Lake Trio. Given how they helped mankind in the PoKéverse, it makes sense to me.
7. God and his Creation Trio. Since every region worships them to some degree after Gen 4, I'm sure Nintendo can find a way to add them. So while this lot or the Lake Trio might not be in the game, hopefully Nintendo added some mythos with them.
8. Darkrai & Cresseila. Nightmares & dreams play a large role also in Celtic lore.
9. The Tree of Life (X game) and the Wings of Death (Y game). Since these two control life and death basically, I feel it would be appropriate to have them.
10. And finally, Diancie. Diamonds play apart in the attire of Royal the world over. And based on what the world have seen of with some of Galar's town, wouldn't be surprising if one turns out to be a mining town, which by the way have been common since Gen 3 to add at least one town linked to mining in each game.