Well , try to not put ennemies with railguns everywhere , we must be able
to survive/fight them.

Like in Resident Evil 4 , there where some ennemies with railguns , very strong
armor, and some hits could kill you, but they where slow on moving/rotating
so you had a chance to kill them.

You can also put some original/futurist flying ennemies , casting little grenades/bombs (in quake 2 you had flying soldiers).