First off, why is remaking of the old zones is important.
I have no illusions: there is a budget, there are workplans, there are resources which a limited, and in the end it's all about money. In the end the much needed remake will come out only if it's financially relevant. It is. I don't have the right numbers, but you (ZeniMax) do. I'm pretty sure most players of this game didn't even buy a single DLC for various reasons. The base game by itself is quite numerous and diverse in locations and content, after all. I don't need to tell you how important are
first impressions. I know that each release you try to steal new players' attention with base game + newest chapter deal every year, but still a chapter is only 1 or 2 zones, and after that players are left with enormous base game. It's only natural if they are drawn to it. Most of Tamriel is in the base game.
And what do they see there? Blurry textures, clumsy animations, bland locates, lack of visual detail, reuse of the assets almost everywhere (Narsis, Hlaalu capital, is built in Indoril style), legendary cites Daggerfall or Mournhold looking like poor villages. I'm sure you know better than me how you can make those oldies shine with the resources and experience in worldbuilding you have now. Rare case of the established-fact-which-is-true is that even the first of yours post-release zones are the sights to behold.
Now, let's get to the zones themselves, and what I would change.
Ebonheart Pact: Dunmeri Lands (Stonefalls & Deshaan & Bal Foyen).
(first screenshot is mine, second is from UESP)
Those locations are a perfect example of everything wrong with base game TESO. Fanfic in the worst sense of the word. It provides familiar fan-service things like volcanoes, kagouties, netches, weird plants, ashlanders and ordinators, adding NOTHING NEW (well, new plants - yes) to the picture and even removing many things (bonelords, silt striders, bonewalkers, kwama foragers). You had to put your creativity in Vvardenfell, substituting for the mainland Morrowind, introducing nix-oxes and hive golems and others. It's good, because it's good to be bold, throwing new ideas into already established thing.
So if you remake Stonefalls and co. - be even more bolder. Giant butterflies. Centipede express. More weird insects and bugs. More psychedelic-looking plants. No one would come screaming at you with canon wars. If was "your" zones from the start.
Bethesda didn't ask anybody when they put dragon mounds and dwemer ruins on Solstheim remaking it for Skyrim.
Ebonheart Pact: Nord Lands (Eastmarch and the Rift).
(screenshot is taken from UESP)
While base game Morrowind took too little of TES III worldbuilding philosophy, Eastmarch and the Rift suffer from the exactly opposite - taking too much from Skyrim's own flows of shallowness. Giants, draugrs, mamoths, bears, wolfs and that's about it. It also hurt a lot to see the same city assets literally everywhere, but it's a global TESO problem.
Again, be bolder. More original creatures like grahls. You came with a GREAT IDEA of sea giants which made Skyrim richer. Go further. The world we see, locations and animals we encounter are only the grain of sand of what is out there in 1:1 Tamriel, correct? Should be thousands of types of animals we couldn't see, in Skyrim alone. As an example of what can it be I'll just throw here a woolly rhinoceros. They are cuties.
Aldmeri Dominion: Auridon.
I remember before Summerset announcement we talked about how Auridon is so extremely lackluster in any way possible. Architecture, culture, flora and fauna. If only we knew back then what was cooked on the main island itself..
Imagine that: there are no unique animal/monster type on Auridon, not a single one. Wolfs and bears, and people call High Rock boring.
I will be bland: I hate Summerset and Auridon with all my soul both and believe they both deserve complete remakes from the ground up. Now, I'm not naive and don't really wait for that, but Auridon can and should be updated. It's also could be the last opportunity to do more lore on the Altmer (something different from Summerset art direction, too). You well know we like you for many things, but I never saw a lorebuff who actually liked your Summerset.
Put more fairy on Auridon.
Less green vegetation, like it was in Summerset trailer.
More magic in the world, damn it. Vivec City has a meteor frozen in place. Screams MAGIC. Nothing like that in all Summerset. You REALLY should've kept that idea with magicka pipelines. Use your own concept on Summerset for inspiration, what I can say. Shimmerene there really shimmers, for starters.
Aldmeri Dominion: All of Valenwood.
I'll be brief on this one, as there was a great thread
here already on the potential Valenwood remake. Current Valenwood is having none of what it should be. It's very-very uninspired. I'm sure you see that yourselves.
It's fauna also a complete miss.
Wild Hunt monsters are the perfect opportunity to be original and flex your art design muscles. Here is a picture of a Wild Hunt monster by Darya Makarava.
Last one, and the most important to me personally.
Daggerfall Covenant: All of High Rock (Glenumbra, Stormhaven, Rivenspire, Bangkorai).
My screenshot of Daenia's forest. And people say there are no good forests in vanilla!
I can say thousands of things, but it won't be necessary. You've made the Systres this year, and there was a right vision of how High Rock should look and feel. Geography, flora and FAUNa. Good stuff.
Now, High Rock presents challenges of itself.
Remove the hell away ayleid ruins. No one likes them there. Not a single person. Not only they hurt High Rock's uniqueness, their presence hurt Cyrodiil's uniqueness as well. Not to mention this type of dungeon lacks identity in itself in most cases (thanks, TES IV). Again, more diversity in animals and monsters, terrain and visual. Only lazy by now didn't say
we need Direnni ruins. That's the most obvious one.
Rivenspire can have more mists and aggressive sharp cliffs. Bloodfiends are creepy, they are a lot like zombies from "28 Days After", and it's a compliment. Oh, and please, don't build up everything there on vampires - it'll make the location one-dimensional, obviously.
Glenumbra - more forest area with enchanted thickets like you had on High Isle. Also, giant sea monster,
leviathan Ithguleor, swimming up from time to time being seen from the shores, will work magic on worldbuilding (if you won't make him a mere killable boss, of course).
Stormhaven has dandelion seeds flying in the air. Good, but it's a single thing. How about unique weather events, like the blizzards you have on Mor Khazgur battleground map? Heavy rains to justify the name of the region? Again, that's just one idea from the top of the head.
More magic-oriented people/society with heavy magic integration.
Your own piece can help you here.
More magical monsters. There are a lot of extremely original monsters in European bestiaries.
Amphisbaena/ouroboros is what comes to mind first.
Tylwyth Teg from Welsh myths.
Nuckelavee from Scottish myths.
A lot can be said about other zones I won't cover here, I'll leave it to others.
Shadowfen, at the very least, can finally have underground worms transportation implemented. And also feature kothringi & lilmothiit ruins - lore fuel that strong no other single zone not from Argonia can have.
And, finally, seeing so much whine here on the forums I want to say: ZeniMax, I love your work, I support you and I want you to succeed. Be more bold, be more original and continue to improve on your creation.
I want to hear what other players would add to the old zones should they be remade, from worldbuilding perspective. Let's try to stay on topic and not go into complains - would be nice to hear ideas instead, art references, inspirations, et cetera.