This thread has got some long dormant gears in my head turning. I am just doing a bit of an idea dump here while it is still fresh in my mind. It might be able to provide a theoretical solution for the extra buttons (or perhaps less buttons)
While I really like the ability OpenXcom to use a variety of resolutions, I've always been bothered by the fact that the user interface does not scale with the resolution. Fair enough as the original UI graphics were built for the good old 320x200. But you do end up with these really tiny buttons at higher resolutions, and I'm honestly not a fan of any game interface where I have to squint to see what I'm clicking. Just exaggerating a bit here as it is not that bad with the X-Com games.
I don't know if this will line up with the direction intended for OpenXcom, but the idea I've been entertaining is a sort of custom UI system. It would work in place of but not completely replace the original. That needs to stay especially as a fall-back interface in case things go wrong.
The idea is to have a user interface text file where the user can define their own buttons, information displays, button groupings (TU reserve buttons). Specify how they look ( from the default graphics set or from a user supplied texture file), their dimensions, whether they scale with screen resolution, etc and most importantly link them to the relevant game functions. Then populate the screen with the instances of the buttons and where they appear on screen.
The person designing the interface would have to take care that all necessary functions are displayed.
One important consideration would be to define the button area. The classic user interface gutter along the side or bottom of the screen, or use the entire screen and allow floating buttons and other elements that can be positioned anywhere. Not necessarily at run-time unless you want to throw in grab handles as well (think: Ultima 7 or 8 inventory screens or Planescape Torment's radial menu). Although that might be overdoing it a little.
Something like this would potentially allow for users to build custom UI mod packs for their chosen resolution or device. It would provide an easy way to introduce and try out new interface elements with different ways of presenting information. Compact unit stat displays for example, shorter or vertical bars, or even do something unusual like a radial percentage clock, etc. But I digress.
