Hello mercy, there are a lot of issues with IP but i think it should boils down to this
The IP of the game (XCOM) includes the names of the aliens, and all the artwork done by the original creators, no limited to the GUI, the specific Tech tree, weapons and other artwork. As well as making something to mimic XCOM (i.e. clone) too closely. A game similar to it in the same genre can be made be it 2D or 3D so long as it doesn't use or copy any of the artwork or IP content.
from your statement
of course the outlook/graphics could change enough not to resemble so much of XCOM
it would be OK.
For example the soldiers and they gray jump suit is basically an XCOM staple, so if you create a soldier with any thigh other than the gray suit you should be OK.
DISCLAIMER: I am not your IP adviser nor do i intend to be, nor do I pretend to know anything about IP. To find out about proper IP usage contact your closest IP attorney and get his.her advise,
my opinions are my own yes only mine, and i like it that way.
