A quick note to start - We already have a set of bosses that specialize in a certain stat, the
Stat Trainers from Sinnoh.
This is a subject that I've put some amount of thought into.
I've been of the decided opinion for a year or two now that this is a good thing to do, but that if you're going to do it, you should do it
right. Don't half ass it. Or at least, you can't half ass it and still consider yourself any better than a monotype specialist. For example, Pokemon based on animals or on objects are categories far too broad. Maybe a team of Pokemon based on particularly iconic zoo animals, or animals from different parts of the world; but you can't just pick 6 random Pokemon from the set of "Pokemon based on animals" and call it a theme.
I'm going to continue to talk for a bit about what NOT to do, since again I've given this matter some thought, and then get into some positive ideas.
Pokemon Uranium does this, but I don't think it does it very well. I've seen six Gym Leaders in that game so far, four of those have a theme that isn't monotype, and of those four I feel that only one of them does it right; the second Gym Leader, who has a "cave" theme. He has two Ground types, and Rock/Steel type, and his ace is a bat-like pure Poison type. This is fine. The other three I've seen, I don't like. One thing that I feel you need to watch out for is that if you're going to "not have a type theme," you need to actually do that. The third Gym Leader in Uranium is supposedly "beach themed," but three of her four Pokemon are Water type; that's no good, especially where there are so many non-Water type beach-themed Pokemon already in the main games. Similarly, the fourth Gym Leader is supposed to be themed after "technology" and generally being a smart guy, but he only has one Pokemon that isn't a Steel type. Now in this case the game at least admits that he's mostly a Steel type user, but it's still not a good example of what you're asking for.
Another thing you need to be careful of is to not reverse-engineer the Pokemon available for your theme based on what Pokemon you want the boss in question to have. For example, I wanted to make a swamp-themed boss for the game I'm working on, and I tried to make a point of not creating a new swamp-themed Pokemon to be found in the area just because I wanted my boss character to have a Pokemon of that type combination. I feel that the sixth Gym Leader in Uranium is guilty of this. She's supposed to be a Gym leader who uses both Psychic types and "illusion" or "mask" themed Pokemon, and one of her Pokemon is a Fairy/Fighting type based on a jackolope, which I feel has no reason to be there other than that its pre-evolution is found in the area; and I highly suspect that what happened here is that the devs wanted her to have a Fairy/Fighting type as a "gotcha" to players who brought Dark (and to a lesser extent Bug) Pokemon to this ostensibly Psychic-type Gym, and then justified it by putting that Pokemon in the wild nearby. Now having "gotcha" Pokemon based on the player's expectations is fine, but for a supposedly "themed" team it's a cop out.
With all of that in mind, I'd would be wary of deciding that
every Gym Leader in a game is going to have a theme other than a single type, because I would question whether you actually have eight good ideas. A not-so-good idea for a theme, like those described above, isn't necessarily
worse than a monotype specialist, but I don't think it's much better either, and if you come across as patting yourself on the back for having supposedly more interesting themes (as I feel Uranium somewhat does) it would annoy me, especially when I think that this kind of thing can be done so much better.
I also think that one should be willing to have two-type specialists - without necessarily pretending that their theme is any more complex than that - if the opportunity calls for it. For example, in my game, one of the Gyms is located on top of a mountain, and I debated for a while whether to have it be a Fire or Ice type Gym, before I came to my senses and went with the El Paso solution - why not both? Another good example of this IMO is
Anastasia from Pokemon Insurgence, who is engineer/technology themed and uses Electric and Steel types. Unlike the mostly-Steel Gym Leader from Uranium, she has a better type composition - two Electric types, three Steel types, and one Electric/Steel type - and I think her team is about as good as can be expected without using new Pokemon (which Insurgence generally doesn't for its Gym Leaders). The only change I would make would be to swap Klefki for Klinklang, to be more on brand.
Finally, relating to the point about how many good ideas for themes your game actually has, I think that one should consider the question of whether non-monotype-themed specialists are better off as other types of bosses rather than Gym Leaders; since you can get away with a monotype specialist more easily for a Gym Leader than for other bosses, and so this approach might enable you to have more bosses in total. In my game, the only Gym Leader who isn't a single-type specialist is the Fire/Ice one, and while I COULD make my beach, swamp, or desert specialists be Gym Leaders, that would just mean removing some single-type specialists and having fewer bosses overall.