Well, you kinda already can.
You can use switch-lan-play while simultaneously blocking Nintendo's servers, that way you can play "lan mode" games like Splatoon 2 or MarioKart 8 DX.
If you legitimately purchase the games you want to play online, you can restore a "clean" NAND backup beforehand so you're running a stock Switch, and there will be no reason Nintendo can see for banning you (it's a bit of a more complicated process than that, but the point is it's possible.)
In the future, when PROPER emunand support is released, it will be much easier to hide your CFW usage from Nintendo, which will make playing online with your LEGIT games much easier (assuming you haven't already been banned).
Or, you can purchase a second NAND chip for your Switch online, keep one Stock and use the other for CFW. Swap them out as needed.
As far as playing pirated games online? It's likely your only solution will be LAN-based, like switch-lan-play.