nv Mega Glalie and Mega Gardevoir are certainly viable. They kind of lack switchins and pretty much have their own role. They both can Trick Band Specs to screw up walls. They're just really underestimated I think.
As for Mega Aerodactyl (which you didn't list), it can be a nice Mega Mence check.
Glalie-Mega @ Choice Band
Ability: Refrigerate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly/Naive Nature
- Ice Shard
- Return
- Earthquake
- Switcheroo/Explosion/Freeze Dry
252 Atk Choice Band Mega Glalie Ice Shard vs. 0 HP / 4 Def Mega Salamence: 300-352 (90.6 - 106.3%) -- 37.5% chance to OHKO
Adamant will get you guaranteed KO, though Jolly might be better for multiple purposes. If Mega Salamence already get even a little bit of Double Edge recoil (which is the standard Ubers move), it'll be dead. Also calc on Freeze Dry vs non SpD invested Mega Slowbro.
0 SpA Mega Glalie Freeze-Dry vs. 252 HP / 24 SpD Mega Slowbro: 206-246 (52.2 - 62.4%) -- guaranteed 2HKO.
If you don't like Freeze Dry on Band set, that's fine, but this move makes Mega Slowbro not able to switch in.
I'd have to disagree. It's still a 780 BST poke that can hold items. The reason that they're banned is, if they're allowed, there would be no reason not to carry them. Which means they kind of limiting teambuilding. Also, it's... actually still too strong when you compare to other megas, just because it gets SD, DD, Extremespeed and Vcreate, which means it's basically unwallable AND near unrevengable. Basically the same reason it was banned from Ubers still apply here, because the new megas doesn't offer new counters and checks to this thing. Maybe Mega Aerodactyl but, that's it.
runbabyrun regarding Soul Dew, I'm kinda on fence on this one but I'd say unban it for a while, because I haven't seen any of the Mega Latis do any good. So we don't actually have any theory OR proof that say they're broken. That's why I'd say test it for a while and then hear what the community says.