Johannes
Arcane
All Caelums have great troops, especially in a team setting. Flying is the best thing you can have in a teamgame.EA Birds best birds - skele spam, usual thunder spam, excellent eagle kings. And bird troops suck both MA and EA, LA has decent troops but the mages suck.
EA is the only one that has usable sacreds so they have that going for them, MA/LA otoh have Storm Guards who are ridiculous in cold dominion.
This is totally wrong, but there's the caveat that a mediocre player can't get much mileage out of them. If you don't have a player who knows how to play Sauro in advance its not worth it. They have great research, and in many ways best skelespam in the game.EA Sauro has trash troops and weak magic if you ask me. Not a fan of humans + death. You want a cheap nerd nation there are many better picks.
Note that MA Pan works totally fine without a bless - white centaurs are good but normal centaurs can do the same job just as well. So going a non-bless build is something to consider. There's no reason to rule out strong non-bless nations like MA Ulm, non-EA Caelum, Sauro, MA Pythium, and so on. Actually they might be preferable here since less people might want them due to being already locked into bless nations only.
Most important thing in a team game is to have synergy between the nations - not just in temp scales and in possible bless, but in that they give you different armies that an enemy can't plausibly counter all three nations with the same army/scripting. And different movement types - stealth, flying, sailing, trapeze/teleport...
Fake edit: so MA Vanheim is in, that's nice. Pretty much keeps the options totally open, whatever you personally want to (and know how to) play, really. Are water nations available? Depending on the map one of those could be an obvious choice.
And remember you have currently no S, and barely any F/D. Filling those gaps should be a consideration, but not a must.
Edit: Gath seems like a great choice here, brings you full magic diversity, SC chassis, giants and rec-anywhere sacred, plus some of the best communions in the game. But it kinda forces you into E4N9 build, which isn't bad ofc, but might not be what you want to play. Also better hunters who can feed your other nations slaves.
Personally I'd go with Gath (E4N9) or MA Caelum (scales).