Outnumbered Effect (Not a buff)
Outnumbered is an effect that is applied each scoring tick (every 5 minutes) in World versus World if the player's team are greatly outnumbered by enemy players on that map at the moment of the tick. It serves both as a warning as well as a compensation.
https://wiki.guildwars2.com/wiki/Outnumbered
Just quoting this to make it clear what the purpose of the Outnumbered is, it works well for what it's supposed to do.
But if ANet where to try to change it into an actual “buff” instead, here's a summary of some of the things I've suggested in the past.
These changes will make each outnumbered player be able to do more defensive work, without creating differences in straight fights (1vs1, 5vs5 etc).
Example, make the Arrow Cart more of an anti-group machine. Give it bonuses the more players it hits, give its main skill some soft CC like slow/chill etc to constantly re-apply. Make one of the skills boon strip 5 or something. It won't STOP a zerg, but it will slow them down, force them to move around, not all stand stacked up etc.
This is a pure wish that ANet changed the way Outnumbered was calculated, from a per map basis, to a per territory area instead. Even more granular would be better, but probably even more unrealistic to hope for. As long as it's based per map, there's a lot less things that can be done to it.