Honestly that's the one thing I don't want. Give us a new Pokemon that goes Bug/Dragon (or give it to M.Yanmega).
What I'd love to see for Flygon line is the dragon living in different environments affecting it's /Ground typing in other ways.
Maybe a tropical Grass/Dragon Flygon.
A icy mountain top Ice/Dragon Flygon.
A volcanic Fire/Dragon Flygon.
A tempest storm Electric/Dragon Flygon.
The thing is Flygon is the "Mystic Pokemon" and referred to as a 'desert spirit's in alot of Dex entries, so I think giving it a chance to be called "the Forrest spirit" or "the tempest spirit" and what not would be really fitting for regional variants.
You can have a "forest spirit" that is bug type, as both grass and bug are fitting for this role. Moreover, bug is simply more logical than grass IMO, mostly because of one thing : Vibrava is already a bug type. From an "evolutionnary" perspective (which is what regional variants seem to be all about), it therefore makes a lot of sense that it would keep its bug typing upon evolving were it to live in a forest. Vibrava could also even lose its ground typing, if the whole line is to get regional variants.
And this wouldn't prevent a Pokémon like Yanmega from getting a dragon/bug mega in any way. Not to mention that "dragon"fly (from a language perspective) is only a thing in English and maybe a few other languages. In most of the languages I've looked (japanese, french, german, spanish, etc), there's no link made to dragons from the name. All in all, if there should only be one dragon/bug, Flygon (regional variant or mega) seems like a more fitting candidate than Yanmega.
This concept was really underutilised in Gen 7, so I really hope they introduce new ones in Gen 8. Majority of them should be from other gens than the first one (Gens 2-5 especially). Some (mostly random) ideas:
Gen 2: Noctowl (Psychic/Flying), Ariados (Bug/Ghost), Granbull (Fighting), Ursaring (Fire)
Gen 3: Breloom (Poison/Fighting), Zangoose (Ice), Seviper (Psychic), Torkoal (Fire/Water)
Gen 4: Luxray (Dark/Electric), Roserade (Grass/Ghost) Drifblim (Ghost/Fire), Gliscor (Steel/Poison)
Gen 5: Swoobat (Rock/Flying), Krookodile (Water/Dark), Darmanitan (Ice), Lilligant (Grass/Fire)
I wouldn't mind if there were a couple of forms for Gen 1 Pokemon as well (I'd personally like to see Ekans (Water/Poison) and Ponyta (Fairy/Flying) lines as well as Nidoking (Poison/Fighting), as long as other generations get more.
You can see some of these are indeed totally random! But I do like the idea for Noctowl (how it isn't already psychic is weird to me), Granbull (has the appearance), Breloom (maybe grass/poison would make more sense, but either way, it would be cool and could even get an exclusive priority or multi hit poison type move), Luxray (has the lore), Roserade (which is really an amazing idea that would stem from the idea of a dead rose I guess), Drifblim (would require levitate to have that typing) and Krookodile (they just moved from the desert to a river, basically).
And regarding Ponyta/Rapidash - I think it would be really cool for it to get a true unicorn form, with all the lore behind it (miracles, magic, etc.). In fact, it could even get "revered" to as a pseudo-mythical Pokémon in a new region, with maybe an area that emits strange powers that, on rare occasions, cause Ponyta/Rapidash eggs to produce this flying/fairy type Ponyta instead of the typical fire type. I think it would be a very original way of introducing a new variant, and I would even go as far as saying that I wouldn't mind if it got its BST boosted to 550 simply because it's a freaking kawaii unicorn.
This will probably not a popular opinion but I think one of the pokemon who already has an Alolan form may also get a regional form here too. Something to make them the face of regional forms as there is a different one in each region. If I'm right on this I would predict Rattata (and by extension Raticate) being the ones to do so as that saves them having to design another early normal type from scratch each time.
I don't see why it should be an unpopular opinion, because that's actually a very great idea regarding Rattata. Rats are known to be invasive and to adapt very well. And it wouldn't necessarily mean they wouldn't have to design new early normal types from scratch in every generation. They still had Yungoos and Gumshoos in SM, and with Rattatas coming out at night to avoid them, they gained a dark typing. I could definitely see Rattatas gaining poison typing or ground typing in other region. Maybe even fighting or water (because sewers).