How about let the killer get a 5% bonus speed if there is 2 player swf, 7% if there is a 3player swf and 10% if there is 4 player swf?
I play this game solo 100% and when I play as a killer it basically full team everytime. Fair? no
When I play as a survivor I know I'm dead at first hook, everytime. No help from swf guys
Another option would be for the survivors to be unable to use perks if in team