I'd love to see a Bug-type regional form. It would likely just be for currently existing Bug-types, since giving the Bug type to pokémon that don't already have arthropod design elements would be kinda weird.
However, there are a few non-Bug-type pokémon in the Bug egg group that could be plausible candidates to gain the Bug type with a regional form, like Gligar, Drapion or Flygon.
As for new type combinations for forms of current Bug-type pokémon, I'd like to see either more Bug/Ghost and Bug/Psychic pokémon, or the first Bug/Dark pokémon.
With regard to the pokémon already shown in trailers so far, here's a list of Bug-type pokémon I think could get a Hisuian form, along with plausible type combinations:
- Venomoth - a good candidate for a Bug/Ghost or Bug/Psychic form.
- Ledian - Bug/Fighting would be fitting but might be too similar to regular Ledian in concept. I could also see a Bug/Fairy form being plausible.
- Ariados - another good candidate for a Bug/Ghost form, as well as Bug/Dark.
- Masquerain - once again, another good candidate for a Bug/Ghost form. Think Masquerain, but with gloomier antennae. However, I don't think a Bug/Water form would be likely (that's what regular Masquerain should have been imo), in the same way I don't think a Poison/Bug form for Drapion would be likely.
- Durant - A Bug/Ground type to reflect a past form of Durant that had not adapted to Heatmor yet.