Terraria is not built on having different worlds like this. I'm not sure the engine could handle it.
If it is a whole new world rather than a branch off an existing one, it seems a bit too, idk, layered? Terraria is a fine length; no need to stretch it.
I don't think they're trying to extend the endgame much, but rather break up the boss rush feel it currently has with exploration.
Addressing the idea, I think the exploration aspect is much needed, but having it as a separate world wouldn't be ideal. I'd rather the biomes turn into their endgame counterparts upon the cultist's death, and are reverted after the equivalent of the pillars are over. The cool background effects could be reused too!
The pillar monoliths could also change the appearance of the blocks to match the endgame biome counterparts too (e.g. the solar desert sand is orange or whatever).
The enemies could be the pillar enemies but with a reduced spawn rate.
Now that I think about it, the idea (with the variants of existing biomes) is probably the best "fix the endgame" suggestion I've ever seen, as the devs can reuse almost everything.
Besides, I despise the pillars right now, and even if you don't mind them, I don't know anybody who really enjoys them (please tell me if you do, I want to hear your viewpoint!).
As for the lunar forest part:
Luminite is post-moon lord but so are the lunar pickaxes/drills, so you'd have to have some weird intermediary mining tool (as well as make luminite stronger because it can currently be mined with a copper pickaxe), and even then, you'd need post moon lord enemies so it's not just boring. I think luminite is better off as a drop from the Moon Lord.
For the lunar forest aesthetic, Vortex is basically the same thing so I think it would be better off as a black void of sorts.
In conclusion, I think the basis of the idea is great, but the details could do with quite a bit of tweaking.
