u/Teto-

There are several communities of old games that are alive and can offer improvements, sometimes necessary ( widescreen, windows compatibility or servers ) and new content, whether they are revival, remaster or campaign extensions, but they are not on steam and therefore often little known.

Here are a few, don’t hesitate to share what you know.

Command & Conquer

CNCNET : The classic versions of Red alert 1/2, C&C, Tiberian sun, Yuri's revenge, Dune 2000
Very populated, offer the original experience.

OPENRA : Red alert, C&C, Dune 2000
Lot of QoL for modern expectations

Dawn of War II

Elite Mod : Multiplayer mod, adding new factions, units, maps

Tzar: The burdern of the crown

Tzared: Classic experience for skirmish/multiplayer, playable directly in web browsers

Battleforge

Skylord Reborn: Revival battleforge, campaign, skirmish, multiplayer

Battle for Middle Earth II

Age of the ring : Single & multiplayer content for BFME

Age of empires online

Project celeste: Revival project of Age of empires online singleplayer + multiplayer, new civilizations

Worth mentioning:

Warcraft III

Hive: Thousand of mods for warcraft 3, new campaign, new game mode, new race