• Implement version checking (MAJOR.MINOR checked, unless 0.x.y or prerelease in which case whole string checked)
• Make sure mods can opt out of version checking (with proper "side" etc.)
• Kick vanilla clients out

PSA: having block/item adding mods on the client while connecting to a server that lacks them "scrambles" the clientside ID registry, causing small-to-severe ID mismatches and rendering the game somewhat unplayable, especially on larger modlists