If someone was using Rock types to combat the Bug types, then most likely Leavanny would handle it.
A Fire type could be handled by Dwebble.
All depending on whether or not they are dual types or not.
In the end, it really doesn't matter though, whoever you feel is easiest, it's because you didn't make the battle challenging enough for yourself. If you're going to be using type advantage most likely most battles aren't going to last long. As well as various other optional things you can do to make the battles easy which in the same way are optional to make the gym harder by not using certain methods.
As for the matter of more new Gym Leaders, I doubt we'll have anyone else replaced, Cheren is replacing Lenora, all together we have three new Gym Leaders which is more then we've ever gotten, I think it's fine how it is, and I'd rather there not be any more new Gym Leaders.