Mostly the changes that come with classic automatically (better paced content releases, consistent tuning instead of weekly panicnerfs) and changes they made to Wrath already (removal of lFd/r systems, shared lockouts between normal and heroics).
Dragonsoul, just like TotC, also needs to get some RP skips to be enjoyable.
And if they want to super convince me to come back, they can restore a bit of the lost content.
Should make for a short xpac, but as far as I remember, most of Catas content was actually decent to great. It just didn't have the polish it needed.