Personally, I would much rather have Fire/Dragon anyway, but I think it's worth noting that there is an alternative interpretation of Astrolotl's movepool. We can definitely circumvent the apparent
obligation to be Fire/Dragon - it really is just a matter of whether people really want to do that.
Starting in Sword and Shield, when a pre-evolved Pokémon learns a move that its evolution doesn't, the evolved form still retains the move - it's just moved to level 1.
As far as I know,
these replaced moves are indistinguishable from Heart Scale moves (look for yourself with Growlithe and Arcanine!) - which means we can use
Magic Coat and
Baton Pass to replace
any two moves of our choosing in the level-up pool for the pre-evolution.
Based on existing examples, we have two options:
- replace two moves, with Magic Coat first and Baton Pass second
- replace one move with Baton Pass only
Since we also have the option to trim the end of the movepool, we can end up with something totally Dragon-free that abides by Generation VIII movepool norms:
- Dragon Rush and Flare Blitz can be removed from the end.
- A late Dragon Pulse is apparently fine, but that and Healing Wish can also optionally be removed.
- We otherwise only have two Dragon-type moves, which is the number we're allowed to replace.
By cutting out all four of our Dragon moves, we
are left with a more-or-less-plausible movepool for a pure Fire-type.
(I think losing Fire would be far harder to justify, personally - I don't know if movepool conventions would even allow us to replace Ember, for example. We also have no more than one damaging level-up move of any other type, so I don't think we could justify any alternative secondary type.)
In any case,
I definitely prefer Fire/Dragon myself for many of the reasons that have already been stated, but I think it's important to establish the options we have in case other people don't agree.
That aside:
Aside from the movepool stuff, Dragon types rarely change typing during evolution, anyway.
Probably the only relevant precedent for gaining a Dragon type is Dragalge, since Kingdra is a trade evolution and Alolan Egg is a stone evolution and a regional forme.
Swablu, Trapinch and Poipole also exist. Honestly, I think Dragon is on the higher end of types when it comes to being added on evolution!