You can do pretty much everything. The only thing that isn't available is 1.0 (before the game got reworked). But that's because the game was completely shut down and re-worked from the grounds up.

I do recommend the YouTube series "Remnants of a Realm", if you're interested in the leftovers of 1.0 that carried over and the lore behind everything. It's very interesting!

As for everything else: You can do everything you want. You even have the option to do it "synced" (meaning your level being synced down the level it was supposed to be fought at) and with minimum item level / no echo. Or you just do old content for the story & play unsynced (Level 50 content at level 80 for example)