The main point for those kinds of worlds after you "beat it", is to play -> find problem -> solve problem (imo without guides, since you actually spend time engaging with the game mechanics then)
Usually the motivator for finding the problems is making large builds, but if that isnt for you you can also go for those huge "tech server goals" (e.g. I want to automate every single item in the game [you will need youtube for those kind of goals though])
I personally have never played a forever world in hardcore though, so idk how it changes it
In general I find forever worlds really challenging as a whole and much prefer booting up a server to play with friends for a couple months every 2-3 years (with the goal of everyone creating a mega base)