One of the few neat things Nu!XCOM had is that adjacent facilities of the same type got bonuses.
As far as I can understand, coding something similar in OpenXcom would be problematic, so how about simulating it with 1x2/2x1, 1x3/3x1, 1x4/4x1 and 2x2 versions of existing facilities?
Examples: One vanilla laboratory can hold room for 50 scientists; 1x2 can hold 120, 50x2 plus 20 from extra space; 2x2 is 300, 50x4 plus 80 from horizontal and vertical adjacency plus 20 from the new space in the middle.
Question: Can OpenXcom make hangars have more than one plane? Since having three hangars at the top is a common layout for base defense building, it would be nice having a 2x6/6x2 Hangar, or even 4x4 if you are very confident about defending your base.