Of the Pokémon fan games and rom hacks you’ve played which are your favorites? What makes them more fun in your opinion? For rom hacks my favorites are Pokémon Blazing Emerald and Fool’s Gold. Blazing Emerald ups the difficulty while adding a handful of custom regional forms and evolutions. It also improves the overworld graphics a bit. Fool’s Gold replaces every Pokémon in Gen 2 with a regional variant. They’re not small changes either. You can use an ice type Cyndaquil, a ground type Mr. Mime, or even a dragon type Lapras. The ideas vary in how plausible they would be in an actual game, but they’re all creative. They even add some new areas in Johto to accommodate the new additions! As for fan games my favorite has to be Pokémon Uranium. It features a beautiful region based on Brazil and over 150 fakémon. The difficulty isn’t too hard or too easy. The story is also decently entertaining without being edgy (which is something more fan games could do).
For rom hacks I'd say Chaos Black and Dark Sapphire which are the only two rom hacks with fake mons I like. As for fan games I love PRO (Pokémon revolution online) I've been playing that particular game for years off and on and I lurk in the discord often. Plenty of good games exist that I'm not aware I'm sure so do recommend some.