Feel free to post any mod ideas you have in this thread.

To get the ball rolling, some ideas for a Warcraft II balance/improvement mod:
-12 units selection
-Queue units in production buildings
-Units have 10% more hit points
-Heal is an AoE spell (5 tiles like Runes) with a range of 7 and has an MP cost of 4 per HP
-^OR Paladins have an attack range of 2
-Unholy Armor offers 90% damage immunity instead (nerfs hasted+unholy'd DK combo a bit)
-Upgraded archers and trolls are a bit stronger
-Flame Shield has a range of 10 and does half damage to friendly units
-Summoned skeletons are a bit stronger and last longer
-Whirlwind costs 1000 gold and does 50% damage to friendly units
-Runes last 30% longer
-Demolition squads & sappers are a bit cheaper
-Improved pathfinding and AI build orders
-Daemons can be summoned by DKs and last as long as skeletons. Daemons can see invisible units.

Check moddb or nexus, likely some already done. As for what you have listed some are easier, others would require an engine rewrite.

Well thanks for the tip, I've now posted there (mod db) about it.

Yup, didn't find a general thread though (both gog and non-gog games).

Some ideas:

Morrowind/Oblivion: A mod that changes stat growth to work more like Ultima 3, or adds it as an option. Basically, there would be certain spots in the world where there would be a shrine, at which point you could pay money (perhaps make this expensive enough to act as a money sink, especially since Oblivion doesn't really have one). Also, while we're at it, include a place where you can purchase "all the health you should have"; this would change your base Health to what it would be if you started the game with your current base stats.

Daggerfall: Rebalance spell costs. Some of the less practical spell effects would have their costs lowered. The Regenerate Health effect would have the per-level component of its magnitude increased (it's way too low as it is, when you can make a spell that fully heals you every 5 seconds for less than the cost of a spell that fully heals you once).

(I think both those mods would be realistically possible to make; the Daggerfall one might involve binary hacking, but you're only changing numbers, not code, so it shouldn't be too bad.)

Also, how about, for each TES game, utilities like the following:
* New Game +: Allows you to transplant your character to a new game. You would keep all items and spells, except those marked as quest items, but the world and quests would be reset. Things like Fame/Infamy and guild ranks would also be reset.
* Save cleaner: Searches saved games for unused references and deletes them from the save file. For Arena, this can be used to clean up the custom spell list (there's a bug, triggered by deleting multiple spells after running out of room for custom spells, that can permanently reduce the number of custom spells you can have; this program would fix the save after the fact.) Later in the series, it would do more extensive cleaning; for example, in Morrowind/Oblivion, if you make some custom potions, but have already used all potions of a type, this would remove the potion type from the game; this would solve the issue where the first potion with a particular name/effect combination dictates the cost and weight of all others.