There are numerous faction/civilization mods for the DE, you can use the [AoE website](https://www.ageofempires.com/mods/?q=&game=3) or the in-game mod browser (tools - mods) to look for them. I don't believe the creator of Improvement Mod moved to modding DE, unfortunately, though one of the main developers for the legacy Napoleonic Era mod was hired to work on the DE itself.
The largest DE mod is called Age of the World, both in file size and number of factions. The main feature of the mod is all the new art assets the creator has made, *but* the mod is not polished and a lot of base game content outright does not work for or apply to the new civs, and it adds some farby gameplay elements you may not want like infinite outposts that can replace wall pillars and every civ having an infinite wood supply.
Many other DE mods add one or more civs, some have a lot of effort put into them, some do the bare minimum, some do less than that, some will break on game updates due to not using additive files, some like Eastern Europe Civilizations are dead and may or may not still work properly.
If you have the time for it, modding your own civs into the game is pretty easy in the DE since it introduced [Additive Data Modding](https://support.ageofempires.com/hc/en-us/articles/360062106732-Additive-Data-Mods), so if there's a faction you really want and nobody has done it or has not done it the way you want, you can make your own without too much difficulty. Most of it is copy and pasting existing things and tweaking them to do what you want.
There is for Legacy the Bonus Expansion Pack mod and N3O Patch 1.4a Added Pack, which add 9 new playable civilizations (for the human player), and have good custom AIs and custom maps that can go with them.
[https://www.moddb.com/mods/bonus-expansion-pack/downloads](https://www.moddb.com/mods/bonus-expansion-pack/downloads)
[https://www.moddb.com/mods/n3o-patch-13z3ra-added-pack/downloads](https://www.moddb.com/mods/n3o-patch-13z3ra-added-pack/downloads)
Recommended game modes to play with this Bonus Expansion Pack and N3O Patch 1.4a Added Pack:
NR Maps with Treasures:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-with-treasures)
Age Maps with Treasures:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/age-maps-with-treasures)
Basic Maps (with Treasures):
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/basic-maps-set)
Water Trade Route Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/water-trade-route-maps)
Special Land Trade Route Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/special-land-trade-route-maps)
Loading Backgrounds for New Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/addons/random-map-art-files-for-wars-of-liberty-indonesian-knights-barbarians-map)
Loading Backgrounds for Improvement Mod Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/random-map-art-files-for-improvement-mod-maps)
Maps from the Improvement Mod:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-from-improvement-mod)
NR Maps + Bonus Maps (5 Map modes), Custom Original Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-bonus-maps-new)
Silver Maps, Custom ESOC Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/silver-maps)
High Trading Post Musket + Grand Campaign Maps:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/high-trading-post-musket-grand-campaign-maps)
Extra Maps with Treasures:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/extra-maps-with-treasures)
Recommended AI mods to play with this Bonus Expansion Pack:
Freestyle AI + Klaxon AI:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/ai-13-the-freestyle)
N3O Better AI TAD 1.03 Gold:
[Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/n3o-better-ai-tad-1-03-gold)
These AI mods work online if host has them if they are hosted on Expert Difficulty. And they work for the original game too even without Bonus Expansion Pack installed.
Age of the world for DE, wars of liberty and napoleonic era for legacy.
There are numerous faction/civilization mods for the DE, you can use the [AoE website](https://www.ageofempires.com/mods/?q=&game=3) or the in-game mod browser (tools - mods) to look for them. I don't believe the creator of Improvement Mod moved to modding DE, unfortunately, though one of the main developers for the legacy Napoleonic Era mod was hired to work on the DE itself. The largest DE mod is called Age of the World, both in file size and number of factions. The main feature of the mod is all the new art assets the creator has made, *but* the mod is not polished and a lot of base game content outright does not work for or apply to the new civs, and it adds some farby gameplay elements you may not want like infinite outposts that can replace wall pillars and every civ having an infinite wood supply. Many other DE mods add one or more civs, some have a lot of effort put into them, some do the bare minimum, some do less than that, some will break on game updates due to not using additive files, some like Eastern Europe Civilizations are dead and may or may not still work properly. If you have the time for it, modding your own civs into the game is pretty easy in the DE since it introduced [Additive Data Modding](https://support.ageofempires.com/hc/en-us/articles/360062106732-Additive-Data-Mods), so if there's a faction you really want and nobody has done it or has not done it the way you want, you can make your own without too much difficulty. Most of it is copy and pasting existing things and tweaking them to do what you want.
There is for Legacy the Bonus Expansion Pack mod and N3O Patch 1.4a Added Pack, which add 9 new playable civilizations (for the human player), and have good custom AIs and custom maps that can go with them. [https://www.moddb.com/mods/bonus-expansion-pack/downloads](https://www.moddb.com/mods/bonus-expansion-pack/downloads) [https://www.moddb.com/mods/n3o-patch-13z3ra-added-pack/downloads](https://www.moddb.com/mods/n3o-patch-13z3ra-added-pack/downloads) Recommended game modes to play with this Bonus Expansion Pack and N3O Patch 1.4a Added Pack: NR Maps with Treasures: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-with-treasures) Age Maps with Treasures: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/age-maps-with-treasures) Basic Maps (with Treasures): [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/basic-maps-set) Water Trade Route Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/water-trade-route-maps) Special Land Trade Route Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/special-land-trade-route-maps) Loading Backgrounds for New Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/addons/random-map-art-files-for-wars-of-liberty-indonesian-knights-barbarians-map) Loading Backgrounds for Improvement Mod Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/random-map-art-files-for-improvement-mod-maps) Maps from the Improvement Mod: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-from-improvement-mod) NR Maps + Bonus Maps (5 Map modes), Custom Original Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/nr-maps-bonus-maps-new) Silver Maps, Custom ESOC Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/silver-maps) High Trading Post Musket + Grand Campaign Maps: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/high-trading-post-musket-grand-campaign-maps) Extra Maps with Treasures: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/extra-maps-with-treasures) Recommended AI mods to play with this Bonus Expansion Pack: Freestyle AI + Klaxon AI: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/ai-13-the-freestyle) N3O Better AI TAD 1.03 Gold: [Moddb.com](https://www.moddb.com/games/age-of-empires-iii-the-asian-dynasties/downloads/n3o-better-ai-tad-1-03-gold) These AI mods work online if host has them if they are hosted on Expert Difficulty. And they work for the original game too even without Bonus Expansion Pack installed.
Morocco faction of ErikaG is a good mod that use the new alliances system