I think I'm probably over complicating things in my head, but the many different things I need to implement into a simple editor are getting quite silly, now!

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A good gui is only half of the editor.

Button wise, I have four directions, four face buttons, and two shoulder buttons.
(Face buttons are ZXCV or IJKL, whilst shoulder buttons are BN or UO, for those using the default keyboard controls.)

The dpad will be used to move the cursor. The touchscreen/mouse will also be used to plot objects, but I definitely need to consider players who are only using a gamepad.
A button should be the "Draw" button, and it'd be handy to have an Erase button, which will likely be B.
We're left, then, with just 4 buttons for things like filling rectangles, selecting the many different tiles, toggling to edit the title and other such things.

I expect I'm going to have to do a lot of multi-purposing with the leftover buttons!