He could also use that water-shield-like move depicted in his character art if needed. Anyways, I agree. He could easily use Mia's and Piers's psynergy. Diamond Berg would rock. He could probably use some of the unleashes too. I honestly wouldn't mind the canon being a bit blurred if the tradeoff would be a great moveset. As for a final smash, even though he doesn't seem to use djinni, I think a summon would still work. Probably not Boreas, since the Ice Climbers have a similar-enough final smash. I'd prefer he use one of the multi-elemental summons. Iris, Azul, Coatlicue, Eclipse, Ulysses, and Moloch all use Mercury djinni.hm. it is true that we haven't seen alex use much psynergy (teleport, water pillar, heal, frost?) but you could just give him some of mia or piers' psynergy, since they're all mercury adepts. i know that wouldn't be as great, but the main reason i want alex as a playable character is because he's important to the plot and is a really cool character as a whole.
I much prefer Venus Lighthouse, since it's just begging to be a stage, and by default it has awesome music. I haven't really thought about what a second stage would be like. If he only gets one, then I think that it just has to be a lighthouse. If he gets two, the ship could be pretty neat.and as for stages... definitely the lighthouses would work, sol sanctum might, but what about the ships? it could sail/fly around, stage hazards could be enemies or even bosses like kraken or poseidon... or perhaps one of the deserts, or maybe anemos sanctum? dno.
Not Anemos though. The only way to make Anemos properly would be to include Dullahan as a stage hazard who appears on stage and kills players three times a minute. That just wouldn't be fun.