No, the problem here is that in games that have FFA you're allowed to pick up multiple weapons, and with good reason. In FPS games snipers might be good at long range but when the firefight is up close, sniper rifles are not helping so you can change that by switching to a shotgun or something. Splatoon works by giving players different roles, and chargers just work in this case because there's allies who ink turf for you and usually keep enemy players at a safe distance giving you a chance to mitigate your weapon's weaknesses. And remember that in splatoon you'll get splatted allmost instantly from anything, you can learn your weaknesses but if you can't do anything about them it doesn't matter.
The reason why i'm bringing up Tentateks is because the weapon itself is known for being extremely versatile. If you go solo without the ability to switch weapons, you'd want to have a weapon that's good on all fronts and the Tentatek is exactly just that. Blasters, on the other hand become extremely useless because they can't ink fast enough to create enough space for movement. If the enemy has a weapon that outranges your roller, tough luck you're gonna get hard-countered every time. Chargers are the same in this regard, without friendly ink and support they can't stop enemies fast enough and become sitting ducks. Knowing now that enemy ink should be coated over no matter what since it's much more likely to have enemies in it. You can argue to remove the ink mechanic entirely but then it wouldn't be Splatoon anymore.
And that's not even getting into team-specific abilities, ex. Echolocator and Tenacity. FFA just won't work, especially not with 8 players.
You'd have to take so many things apart about Splatoon just to make FFA possible that it's allmost an entirely different game. The closest you'll get it is going 1 vs 1 in a private match.