Eh. Maybe more startup time would do it? I mean, it's freakin' Kamehameha. It can't be too weak. I might have to rethink his other Special Moves if that were how it was done (i.e. give him a new Side B and make the old Side B the non-Kaioken neutral B). It would be annoying to only use it in Kaioken mode though.
And Kamehameha is not a real killer either. It takes at least 135 percent to KO Mario from center stage with it (if all 5 hits connect). That's pretty bad for a move with the startup time of Falcon Punch. And, all you have to do to avoid it is shield, which should be easy since the move is telegraphed more than just about anything else.
Although the range on the move is huge, I'll admit. But, that's kind of the point of the attack - gratuitous beam attack with huge range! You can't really have Goku without his signature move either. I could make the startup more like in the actual Anime, but I'd have to make it stronger, as such a startup would be over a full second (as opposed to the current startup time of 46 frames, which makes it the slowest-to-hit non-charging move except for Dimensional Cape, which hits on frame 52).
Of course, I am reasonable about these things, so if you have an idea of how this could be changed up, let me hear it.