My ideas are:
I think these the campaign build would benefit from having the option of a toggleable grid system with snap to grid.
I would also like to see some kind of top bar where you can click to make the build blocks and the connect them afterwards.
The ability to select multiple blocks and move them at once. For a complex decision tree each one needs to be moved one by one.
Expanding the view towards left as well. Right now you can only extend outside the window to the right.
To me the campaign builder feels very central to Mautic, and would love the builder to get some more love and be on par with modern UX.