Golden Girl

All the new story arcs from I17 onwards ahve been set roughly in the 20-30 range, with nothing added to the 30+ part - which is one of the main reasons I think they're planning a 30+ or 35+ expansion in Praetoria - like they're filling in the "gap" between the 1-20 GR content and a possible 30+ expansion for Praetoria.
Even the new TFs are kinda in this bracket too - there's quite a lot of new content available once people leave the 1-20 GR content - plus, it's also tied into some of the story elements they're familair with from the 1-20 stuff.

I think it might be more to do with almost 7 years of 50s building up, combined with frequent requests for an endgame system, and the form that I've heard players can fill out when they cancel their sub, stating why they're leaving - the pressure for an endgame system reached a point where the devs would be silly to ignore adding one.
The same pressure is going to build for tougher Incarnate content after we get the first 10 slots - only this time, the devs are already planning for that

What about solo content that's an avatar-only unlock?

It's almost like you're caught up in a war or something.

They're getting a little tweak in the enxt patch, which is up on Test right now.

I think the idea is to make Praetoria a looming threat, so we get plenty of build-up to the actual Incarnate content - and I wouldn't be totally surprised if the expansion of Praetoria includes Primal Earth based contatcs that give out missions that take us to the current Praetorian zones, or any new Praetorian zones.
Áll the zones are co-op, Steve Sheridan has already linked up with the Wardens, Arachnos has linked up with the Crusaders, and the co-op-friendly Vangyard are setting up a forwards base in Praetoria - a new 35-50 Paretorian zone set outside the sonic barrier with co-op Vanguard content as well as separate Hero and Villain content might be one possible way of expanding Praetoria and continuing to tie it in with the normal 1-50 path on Primal Earth.

The league UI allows for 6 full teams of 8

That's correct - but the devs were looking at the bigger picture - Praetoria was able to give a new starting expereince, as well as provide the first stage of the endgame system, and cover the idea of side switching-too.
The way Praetoria is set up, game-play and lore-wise, gives way more flexibility and development openings than a revamp of the blue or red side lower levels ever could.

This is wildly incorrect

So far, we've had 5 slots, with one universel level shift and 2 Trial level shifts - so it's quite possible that the next 5 slots will also add another universal shift, and 1 or more Trial shifts - and combined with the massive power boosts the actual powers give us, we're simply going to be too powerful for any kind of non-Incarnate content.
The current 2 Trials are already pretty easy with the 3 available shift we have now, so we're going to need tougher content to match our increased power.

Trials use normal gameplay mechanics, either with a twist, or just turned up to 11 - we should probably expect some kind of escort mechanic in at least one future Trial

It's almost like the game's some kind of multiplayer thing.

But it's a solo activity - you should support it

You'd prefer an escort phase for a Trial?

Which Trial already has the air attack feature in it?

It's likely that Incarnate arcs and missions would be tied into the Praetorian invasion and Tyrant trying to conquer the multiverse.
Think of the Incarnate system as being like a gigantic RWZ, with different options to experience the main story - there's a raid option, a TF option, a story arc option, and a repeatable mission option - but all of them are tied into one big story about an invasion from another dimension.
So far, the Incarnate content has given us a raid option and a TF option as ways to experience the story - and the devs have said they're looking at story arc and repeatable Incarnate missions - there's no real reason to think that these won't be tied into the main Praetorian invasion plotline.
The meta-story has already been written - the lore behind Incarnate powers and the Well has already been laid out - so any kind of content about getting Incarnate power will be tied to this lore and meta-story.

Yes, anything even slightly challenging makes me, rage.

Fixed, to save time

I think the aerial attack mechanic from Sutter and Apex will show up in one or more Trials - it's too much fun not to - and having 20+ players bouncing around to avoid it will be evn more chaotic that 8

Actually, Praetoria was one of the things quite a few players asked about when it came to adding to the lore - the original Maria and Tina arcs left all kinds of things unanswered about Praetorian Earth.