Fire/Poison
Poison
Ice/Steel
I was disappointed to see that Fire/Electric did not make the slate, but Fire/Poison is very similar and slightly worse in some aspects and slightly better in others (Tspikes absorption / immunity and a Fighting-type resist are very nice). It is my primary choice for many of the reasons Fire/Electric was - it has excellent STAB options, a defensively versatile dual-typing that nevertheless exposes it to very dangerous weaknesses (4x to Ground, 2x to Water, Rock, Psychic). All in all, I feel like it would be a typing that would allow us to be much more creative in the rest of process, because it's not abysmally bad to the point where we would need to compensate with ridiculous stats / abilities / movepool, but it's also bad enough that it doesn't fail to meet the concept.
I went with Poison as my second choice because pure Poison is a terrible offensive type but a very decent defensive type. I like Poison solely because it will be a substantial challenge taking the typing in an offensive direction, and I want to see if we will be successful, and also exactly what it will take.
Ice/Steel was my final choice because, out of every other option, it's the only type that's both "bad" while having redeeming factors that will actually be relevant in the metagame. A dragon-resisting Pokemon with STAB ice attacks is a decent niche. I just don't think it's as good of a niche as the two typings above, though.