Hey, I'm recovering from a er, very low sleep night, so I am afraid I can't put all the effort I want here. I'm going to be dividing this into two different categories, that is, a set of typings that are guaranteed on the slate, and a set of typings that I am heavily considering for the slate. I am aiming for about 6-8 total, so just keep that in mind.
Guaranteed:
Dark / Poison: This typing has been widely endorsed by just so much of the community, and for good reason, it simply is very hard to take advantage of thanks to only having one weakness, being Toxic immune, and just being the best Glowking answer.
Dark / Ghost: This is another fairly acclaimed typing that offers similar defensive merits, with only a weakness to Fairy, can completely deny Knock into Close Combat as well as Ghost into Ghost combos, and offers two potentially solid STAB options.
Steel / Flying: This typing is the most endorsed out of every option, boasting excellent switching opportunities thanks to a literal mountain of resists and immunities, and its only two weaknesses are Color Change resists. This really is the best defensive option we have available, although it doesn't really offer us initial STABs (Flying stab moves sorta suck lol), and does have a bit of competition with Corv / Cele / Caw.
Water / Poison: That said, Water / Poison comes close to Steel / Flying's defensive merit, just look at pex to see why, with multiple Weak to CC resist combos, and is one of the few water types that leaves Rillaboom open. Access to initial Water-type STAB is a major bonus given how wide its neutral coverage is (just look at Suicune), and the only like, actual drawback is that this is yet another Water-type that loses to Libra rip.
Potential:
Fire / Fighting, Fire / Fairy, and Fire / Steel: These all offer honestly like the best mono-attacking stab ever in Fire itself, just look at how stupid Astrolotl is. These typings, despite having access to a great initial STAB aren't even lightweights defensively, with Fire offering several Resist/Resist combos to Steel and Grass. That said, Fire / Fairy really has to justify needing HDB over the rest of the slate, Fire/Fighting gives up the valuable Fairy-resist, and Fire/Steel basically shuts off setting up on Libra, which is potentially open for the other two. They all offer a streamlined offensive gameplan with their own specific drawbacks.
Ground / Flying: If you wanted to make CAP 29 justify its place on a team purely based on immunities, this might be your choice; a still valuable Volt Switch immunity almost immediately justifies this typing, and being able to basically freely switch into some of our Ground-types really helps it out. There's valuable access to a Grass Neutral to Resist combo, which isn't really offered by many other Ground-types (Bug/Ground is notable, but arguably just a tad outclassed), and turning Scald into a resist is pretty easy.
Electric / Ghost: This typing pretty clearly centers itself around the dual aspects of just making Tomohawk incredibly sad, as well as several other flying types, and having a fairly good initial stab combo. Aside from those two compelling aspects, it also has a load of Neutral to Resist combos including Grass, Fire, and Water, which really actualize Color Change, and can technically set up a Dark Weak to Resist combo, though doing that with Knock Off is sorta wasted.
Water / Ground: This is a super recent submission, but fits into a similar camp as Steel/Flying and Ghost/Dark in terms of just having very minimal weaknesses, boasting a Volt Immunity over them, at the cost of completely sacking the Rilla/Kart matchup. I'd like a bit more discussion here, but access to Water and Ground stab is actually fairly compelling, as are its numerous neutral matchups.
Grass / Poison: This is perhaps the most centered around just beating up Rillaboom, though it offers a fair few Weak to Resist interactions such as Fire, Psychic, and Ice, which really actualize Color Change; just look at how this makes the Krilowatt matchup go. I do worry a tad about the sheer lack of good offensive STAB options, but frankly we can just give it good neutral ones like a lot of these typing may end up going. Overall this is fairly solid, my main worry is just that you're giving up the ability to do Poison resists w.r.t. the Glowking matchup.
Ground / Fairy: This is honestly one that I missed up front, but it makes a compelling case for itself thanks to the sheer utility it'll bring to a team thanks to a great initial palette of resists, and well, the vaunted Volt Switch immunity. Both of its STABs are pretty good offensively, meaning that you get a great bonus for positioning this mon well, and while it can struggle a tad with Weak to Resist combos thanks to the sheer strength of Steel and Grass moves in the meta, it can still access them with Ice and Water to a limited degree. I definitely want more discussion here though.
Again, subs are technically still open, but like, I would still like discussion. I aim to have a voting slate posted by afternoon tomorrow, as a fyi.