There is moving mechanics, close attack, ranged attack and inventory mechanics. Whichever you change it will still apply to any theme.
You could add new actions, like jumping or crawling, and they would again fit in any theme, just like first person shooter mechanics can be made with elfs, commandos or aliens all the same. I can't think of any mechanics/action that would be specific to only one particular theme. If you are not shooting bullets you are shooting arrows or magic bolts, mechanics is the same.
I don't see the problem actually exist. But since almost all the roguelikes use basically the same mechanics, if you can think of something better it should definitively be more valuable than anything else. A theme is a matter of mood, gameplay, controls and user interface is what makes games better or worse.