Having replayed this content recently, I can see why they might want to change it after the Imperial settlements on the Telvannis map. Port Telvannis is pretty dumb now. Getting from the ground to the walkway is best navigated by levitating, which is not so strange for a Telvanni city, but the thing itself is pointless. Some other towns in TR have interesting vertical constructions against a cliff face and such, maybe they should make PT more interesting vertically? Or expand the port so it contains more than just a few river striders and slaves
Like I said, I think the answer is to reduce the footprint but basically keep the same amount of interior cells. Have the ground level be more cramped with a sprawling skywalk up above, which CAN be navigated on foot but is understandably much easier to get around with levitation. Don't force the player to levitate in anything that isn't a Sadrith tower, or doesn't have a character-based explanation. Marog has a spellsword who wants to be left alone, so his house is built into a cliffside without a walkway attached. Stuff like this is fine, but it seems like there's a sort of flanderization of the Telvanni which often occurs within the TES community, where people like to pretend that they're unreachable without Levitation when the reality is that it's only needed to actual reach the House Masters in the upper sections of towers.
Also some of the quest lines are OK to good, but a lot of it is not really fleshed out. Most of the Telvanni councilors are just analogues for the Vvardenfell ones, but with very little unique dialogue for all but one. It kind of stands to reason since the mainland Telvanni are supposed to be even more isolationist. But as I mentioned, unless you turn it off, you still need their support to be appointed Hortator for the vanilla main quest... I can imagine the original team just wanted to release the map and figure out the content later.
None of the quest content is bad per se, it's just a matter of it having been left behind by more modern TR content. Like you said, some of it is also clearly unfinished or just not fully realised, too. The chores for the Mouths are all pretty bland or allude to political machinations which the player has no ability to influence, and there are also balance concerns like the quest for Archmagister Dral where you're basically given an enchanted Daedric dagger just for showing up and speaking with him. The Ranyon-Rhun questline is nice because it gives you an opportunity to do some Telvanni content that is more focused on mundane problems than on accruing personal/magical power (which basically defines vanilla Telvanni), but I'm pretty sure that it can actually be completed by non-Telvanni characters. Mithras is clearly supposed to be a sort of mainland counterpart to Aryon, but having Mithras as your "patron" only makes sense if the TR content is segregated from the vanilla content, which it hasn't been for some time. At this stage, I'd say anyone looking for extra Telvanni content is actually just better off playing some of the additional content included in OAAB than bothering at all with the mainland Telvanni stuff.
As for other Telvannis content, some of it has already been dummied out in recent builds, like the Killing the Messenger quest(line) which gave you two options for completion, either in favour of the Telvanni or the Mages Guild. The last time I played through it, I quite liked the Gah Sadrith Inquisition stuff and how it tied together three different quests in a fairly complex way. However that has its own problems of kicking off a chain of events which might seem fairly innocent, but then ends up involving you having to fight Level 30+ characters and trawling through a large Daedric shrine with an enormous amount of amazing loot. Kemel-Ze is still unfinished and is emblematic of early TR design. It seems like it was made big purely for the sake of being
big. Something like 30 interior cells, many of which are pointless, bland hallways or both, and since it's unfinished, trudging through it doesn't even net you a proper reward, although one is available with no-clip.
Like the Helnim content which is largely being transplanted to Firewatch, I think plenty can be done with a lot of these old quests. They just need to be given proper resolutions or be tidied up a bit. I don't think mainland Telvanni needs a sort of "main" quest as it currently does, and it certainly doesn't need that quest to be handed out to the player by an Aryon clone. Just give the player some things to do that aren't basic chores for Mouths (vanilla already has enough) and some House intrigue to get involved with which acts as a bonus to the vanilla content, especially since most people agree that Telvanni is the most enjoyable of the vanilla factions.
Edit: Speaking of Telvanni content you needn't bother with, I really want to proclaim that Uvirith's Legacy is LARPer shit. It seems nice until you realise that the mod's benefits don't present themselves until the player has become Magister of the House, at which point you should really only be playing for a few more hours anyway. People who play for hours and hours after completing the main objective of a playthrough are mentally ill.
For compatibility and for consistency, if anything.
At first the characters are pretty down-to-earth and there is some day-to-day management of resources involved, but later it adds completely crazy shit. I installed it in my latest playthrough again but haven't progressed with it yet, since I'm doing the TR content. But I remember a piano among other things, not to mention the dungeon with a vast torture chamber...
