It's more the facility events that you want mono teams for but that's largely due to the challenge battle bonuses, and to some extent it being easier to keep them alive for that bonus as well. And for that you don't exactly need entirely stellar units in the right element.
I think the maze thing was datamined forever ago but it's nice that they finally added more about it, which suggests it'll be sooner rather than later. I do hope it's mechanically interesting (and IIRC there was something in that initial info about earning weapons through it?); since events are so boss focused it'll be neat to have something that's probably more about exploring levels... as long as it doesn't get super tedious to run a lot. And similarly it could be a good opportunity to not be mono-element (and the name suggests it won't be), since if it's co-op focused but not a raid/boss, you could do things like have players split up to fight different enemy groups that are fitting for their element, or take the lead for one part due to what spawned, etc. Maybe I'm just getting my hopes up for something that'll just be fairly basic, of course...
in any case, looking forward to tonight's info, though who knows how significant it will be/how far in the future they'll talk about