Respect Your Elders is a mod striving to solve one of my biggest annoyance with Multiplayer Minecraft: single-use minibosses
You can now respawn the Elder Guardians using the Elder Crystal in a Monument
You can now gain access to the Evoker again using an Emerald soup and exploring a Mansion
Note: Due to how minecraft is coded, mansion & monument detection only works on worlds that use an instance of ChunkGeneratorOverworld. So not all custom world gen will work (but most should, compat. table welcome)
Extra: This mod also adds a /structgen command which you can use to generate a real Minecraft Mansion - because let's be fair, they're way too rare. Note that this command is pretty hacky and will only work in the vanilla overworld and maybe some other overworlds (simply due to how minecraft is coded). I recommend testing this command on a copy of your world first!