Rant incoming: love the idea and how it works with RetroArchivements. My only problem, and that's the reason I do not use them, is that with RetroAchievements games turn basically into an always-online game. Especially with handhelds this is annoying, because I do not have online connection on my Steam Deck all the time. And once I was playing a JRPG and could not play it offline, because I did not want to miss the achievement. That's when I realized this is not for me. I would love to have offline achievements.
There are also services by fans to revive the online experience of servers that are shutdown. One famous (and funny name) is Pretendo Network. I never got into these fan created online servers, but the idea is amazing.
Otherwise there are so many great games one can emulate. Also one of my favorite stuff from the emulation and retro scene are Romhacks and Mods. Think of it as Mods for your PC games, but instead for the classic console games. They can take any form, like DLC like new content like characters, vehicles, maps, randomized builds, new graphics or bug fixes, new stories, game modes, or new attacks or skill sets and even simple cheats. Romhacks are a world of its own to explore. In past years I wrote a few articles about this with many recommendations. You just need to know how to patch them (its explained in the article). Have a look as an inspiration to play next: https://thingsiplay.game.blog/2025/01/23/snes-mods-and-romhacks-collection-2025/