Onslaught needs maps for fewer players! Speaking as someone who can play regularly but with a limit of 4 players the maps can be a bit large and with vehicles like the leviathon pretty much a forgone conclusion if one side gets it first and can use it fairly well.
Ideas for small maps
small maps - simple link setups
N1
/ RPC < > BPC
\ /
N1
N2
/ RPC - N1 < > N1 - BPC
\ /
N2
RPC --------------- N1 N3
| /
| /
| /
N2
/ |
/ |
/ |
N3 N1 --------------- BPC
Various vehicle setups can be used on these.
Restricting the types of vehicles for smaller maps make them more interesting.
For example:
No leviathon - TOO big and pretty industructible
No SPMA - just plain boring sat there launching rockets via sattelite
Manta - always good fast and responsive
Scorpion - nice little buggy good for maps like tricky - especially for getting the redeemer. :)
Goliath - nice heavy artillery slow moving but powerful.
Circada - basically when used correctly a flying goliath :D
Raptor - nice little flying vehicle - fast and with a decent selection of weaponary
Hellbender - great for throwing yourself off hills in a fantastic way. Weapons are good and reasonable speed.
Idea for mutator
Vampire for vehicles? not sure if this has been done or not