Free for all modes could be fun, I don't think having a turf war or deathmatch variant would work well with splatoon though, both in theme and gameplay. I could see something like free for all splat zones with multiple zones, owning a single zone gives a small amount of points, owning more than one gives a little more. Something along those lines.
The real problem with free for all would be all the different colors everywhere, there's enough color variety to where I think it could work, but nobody would have enough ink coverage anywhere to get around. It'd be quite the mess.