What are your thoughts on the idea of more fighters learning Kaioken?

To me, the idea is both good & bad.

I believe its a bad idea because if Earthlings uses Kaio-Ken, they will simply injure themselves. If the Kaio-Ken hurt Goku, think about what it would do to a human character. They are not as durable and resilient as a Saiyan. Nor do they get zenkai boosts after battle either. All the Kaio-ken would amount to if used safely is be a small multiplier that won't even get them on the same level as their enemy.

But someone like Piccolo could make better use of the Kaio-ken due to his body being more durable than a human and he can regenerate his cells. So the damage would be tempered by his healing. But he still wouldnt be able to fight god level enemies with that technique alone.

I think the best route is for other characters to learn different techniques, learn god ki, have their potential unlocked beyond their normal limits (which is different from Guru unlocking your own latent energy), and using team attacks.

So I think Piccolo's Potenial Unleashed form & Orange Piccolo are great for him. Krillin's power-up from DBS EP 76 could return after he trains with Goku to understand the power better (or something like that). And Tien, Yamcha, and the others could get techniques like what I thought up in the link below.