I agree that being part Rock would make sense and could be easily tied to volcanoes, but we already got some Pokémon which are already connected to volcanoes. Which are these Pokémon?
There is also Magcargo, even though it seems more connected to magma per se than volcanoes in general. Anyway, out the three mentioned Pokémon, two of them have the Fire and Ground typings (in the case of Groundon, when it becomes Primal Groundon), so it wouldn't surprise me if Litten's final evolution got a secondary Ground-typing as well, if it were to follow the logic behind Camerupt and Groudon.
Besides, Fire/Ground should benefit the Pokémon more than Fire/Rock. Immunity to Electric/Thunder Wave for a potentially speedy special attacker + only two weaknesses (with Water being the glaring one) + neutrality to Stealth rock is much better than a typing which makes it extremely vulnerable to both Ground and Water and vulnerable to Fighting and Rock/Stealth Rock, even though the resistance to Flying could be useful.
Regardless, another Fire/Rock Pokémon wouldn't hurt, especially if it got a useful ability to cover some of its weaknesses.