Ancient Time Portal

Make the ancient city portal a "time portal". Activated by Echo Shards & Chorus Fruit. Echo Shards link to the past (recovery compass). Chorus fruit teleports. Combine and teleport through time. Must fly through it with the Elytra at a min. speed. Explore a whole new prehistoric world. Maybe giant dinomobs and volcanos.

No return portal: this is an echo to the past, not a new dimension. Set a status effect lasts from 5 minutes to 1 hour (not removed by milk) based on how much chorus fruit is used, at which point you are yanked back to present day. Be prepared for the start (stuck there for awhile) and the end trip (yanked back in the middle of what you're doing).

This (or anything similar) creates a new sense of progression greatly lacking in other updates. The Wither, Woodland Mansions, Ancient city, Ocean Monuments, even the new trial chambers are new challenges of things you can go do, maybe get some unique loot, but lack the sense of progression you get from starting a new world and working towards killing the ender dragon. Nothing else really builds towards a goal. This builds on the previous progression (requiring the chorus fruit and elytra). This also provides new functionality without continuing to crowd the current early game progression.

Also plays into the unofficial lore that some catastrophe happened. Current thoughts are ancients were trying to escape from something through the end. Maybe they were also exploring going back in time to prevent the catastrophe.