(4-2) Non-cubic voxels (Unsupported by KV6, VXL nor VOXLAP)
Coding Style
(5-1) Adding more global variables without a strong justification and adequate documentation on regard to their role and thread safety
Other Restrictions
(6-1) Adding an anti-cheat system like VAC or PunkBuster (Commercial solutions cost too much for a nonprofit project, and it's very likely that they're unusable in open-source projects.)