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).