Hi @mike , you can see the Wix list of upcoming features here:
https://www.wix.com/release/notes/new-features
In regards to the sub-sub menu - it is in the process of going through an internal testing review. If all goes well you should have it soon. Apologies that I cannot get a specific date for this, but I can say that the feature will open to Beta account holders before releasing to all, and it seems that is on the horizon at the moment.