To fix the idea of I'm first into something everyone new or old can get a chance at, how about the first player to reach a new world rather than the first player to reach the last tier world, because at least then it's not just a single person ever getting that achievement. Strange Day.
Fight a Titan naked would be cool for someone ridiculous, empty your inventory before fighting.
Depending on how beacons play out, maybe something to point out that you made something popular that everyone wants to visit. The architect.
Learning your roots, discover all the lore, standard have you been playing as much as we wanted you to.
A mob a minute, make 60 kills in a single hour.