I have a suggestion:
How about making this new portal in the forgotten knowledge biome as a mini adventure mode (randomly genearted map), after enter the portal (you can't carry anything over), there is only two ways you can get back, get killed or finish the quest objective (killing the end boss), if you die there, you lost all your stuff there, but if you win, you can get back with special rewards (a new loot or spools to weave skin or someting), and new milestones for the quest can be achieved by completing it multiple times, which will grant you perks for the quest such as a starting weapon, large backbag, food or healing items, etc.
For most of us that have a mega base for thousands of days but don't want to start over to build another mega base, it would be an interesting challenge to venture into an adventure quest world trying to survive, craft gears and beat the quest boss in the small new world and get back. The quest will reset after you die or complete it. I believe it will significantly reignite the interest of old players.
During holiday or special events, the quest could also be hand crafted and made as a public event, and by getting into the quest, doing solo or paired with other random players in a public sever, so that you can't cheat.