I think it would be more sensible with what others have already said.

That is; keep the current class/job system as it is but just introduce anything new from 3.0 as a stand alone job without a class prerequisite. Simply level 1-60 as a Dark Knight.

Trying to overhaul and change the entire system could just lead to more trouble and bugs than it would be worth. I eventually got tired of trying to keep up with all the changes in WoW.