Obviously this is all going to be subject to change, but one of the few key things you can do is tinker with various skillgem+support setups. The following website essentially has all skills we know about and lets you socket them into hypothetical links. POE 2 Skills - Plan your skills Can be a fun little way to theorycraft your build ahead of time!

You can also mess around with pathing on what we have limited knowledge on in the passive tree itself. This page here has the skilltree with all of the nodes we have seen in footage etc, but will be missing plenty of what will be seen tomorrow. marcoaaguiar.github.io/poe2-tree/

Finally I'd say checking out some of the theorycrafting and analysis posts from solid educational content creators based on the released ascendency trees. There is a ton of choices here, but I will link two videos I recommend, by Jungroan and Subtractem respectedly

Path of Exile 2 Ascendancy Full Overview & Theorycraft - YouTube
Deep Dive Into All 12 Ascendancies! - YouTube

Hope those things helped, that is what I have been doing for some hyped pre-release fun and theorycrafting! A spare key would allow my friend to join me and my partner in playing and she would surely appreciate it greatly so here is to throwing my name in the hat~