Use Super for moving window and enable resize with right mouse button

Well, the menu is needed for setting always-on-top and making a window be present on all workspaces. So if resize-with-right-click conflicts with showing the menu, screw it, we need the menu more.

I'm not sure about Alt or Super as window movement modifier; Alt is common at least and people are aware of its existence. Super is undiscoverable, so I'm afraid people will assume moving windows like that doesn't work in Gala.

You get the window menu over the titlebar.

Some apps that need Alt+mouse in quite some cases:
- Blender, for moving the viewport with LMB, used for graphictablets or touchpad
- GIMP, I dont remember where, but there's something with alt for sure
- Inkscape, for selecting objects below others

Plus, we already moved almost any wm related shortcuts to Super instead of Alt, so it's definitely discoverable.

Good, but resizing with right mouse button doesn't make sense IMHO. What are the use cases for it?

Well, the same as moving the window inside its area, it's faster than catching borders.

I wonder if it will conflict with dragging stuff with right mouse button, as seen in e.g. Nautilus.

It will only resize if you press super (or alt), just like with moving.