Currently hovering the mouse over methods shows a popup with the signature.
Quite often I find myself in need of figuring out if a method is public, protected, private, or if it's a slot. This shouldn't be too hard to add.
Additionally it would be quit nice to see the doxygen comments in that hover too, as it's done with Javadoc in the Eclipse IDE. In that case, you should consider adding the dialog/frame feature mentioned in QTCREATORBUG-8054 to the documentation hover.