I'm sort of dragging this out in a lot of posts, but writing it all out helps me think about it.
So after some thought, the team I've decided on so far would be:
Jirachi, Manaphy, Bisharp, Mega-Altaria, Galvantula.
On edge: Heatran, Shaymin, Tyranitar.
The team's a weird combo so far, which is what I like. I'm not going for the most hyper optimal there is, otherwise this group would be some boring standard mix of pure commonly seen OU Pokemon (I am aware that that is a tautology).
Mega-Altaria: The reason I'm going with this is, while it might not be the best mega or fairy type, to me it's interesting. It's also a partial case of, "Well, screw the competitive players, I just use the pokemon I like" mentality.
Galvantula: You're right on the speed thing. This entire team is slow, to the point where a standard, slightly boring Galvantula's sticky web is almost needed as a crutch. Interestingly, in the not-hyper competitive 3DS multiplayer, Galvantula is rather rare.
Heatran: It's wanted more-so to be a wall, but it's 4x weakness to ground makes me question that when everyone and their mother seems to have at least one Pokemon with earthquake as a move. It's not fun being weak to ground.
Shaymin: Shenanigans.
Tyranitar: Seems good, but I'm not sure if it fills a needed niche or not.
The way I look at it, I tend to try for a team that can balance a focus on the stats of Attack, Sp. Attack, Defense, And Sp. Defense.
The current set-up looks to be: Physical/Special Attacker, Special Attacker, Physical Attacker, Mixed Attacker + Mixed Defense, and Frail Set-up.
The on edge Pokemon: Dubious Bulk, Annoyance / Special Attack, and good overall and a possible wall but uncertain.
Then there's the need to examine and holes in type coverage, both offensively and defensively. This is something that I'm terrible at and can't really be evaluated until I make move sets for everything.