Amazing! I also had the same idea!
I began the project of reversing DOOM into XCOM shooter long ago.
The main line was transfering the original UFO UD (mb TFTD) art to doom with minimal changes.
Even if the project is frozen now, I still hope sometime to finish it!
And I would gladly appreciate help!
I had to use zdoom instead of gzdoom, because isometric corpses were cut by the floor in gzdoom.