yes goku is a bit weak, vegeta can beat anything with his big bang, or even final flash if he has time to make it
as far as spawn camping, this could be prevented with the new map sizes, just dont throw all spawns for one team in one spot. making them hidden somewhere would help too.
the solar flare does need help. it only works if they are less than 10 feet away and looking at you, but then it only lasts 20 seconds
kaioken for goku is a nice idea, but it would have to be a toggle; maybe put it on slot 8 so they can pick it and turn it on/off with a mouse click then choose the weapon to use it with. kaioken should boost their attacks according to its level (kaioken x1, x2, x3, etc.). but when kaioken gets to a higher level it should burn ki much faster, which would prevent that level from being used by weak players, which would make them want to build up power level
i found that vegeta with power level of 10,000,000 has endless amounts of ki so a goku at 10mil should definitely be able to do kaioken x50
remember on goku's capsule headed for namek, he did kaioken x20 before he was even at 1mil
another thing that would make goku better is improved teleportation. maybe leave the teleport the same, but enable him to teleport while he is charging an attack; he does this with cell in the cell games and blows his top half off with a kamehameha, but of course cell regenerates and so and so...