There is one answer to this that might be kind of cool...
Okay. Leave the levels, strength, all the stats stuff like ki pool and that type of thing...leave it all how it is now.
What could be added to make it a little tougher to beef with a transformed player is a second less severe melee hit animation. Like instead of going flying helplessly and righting yourself somewhat quicker then normal, you could just get pushed back like 2/3 the usual amount and force, and instead of spinning in pain, they could just kind of be knocked backward some without the spinning. You could even dub in some taunting sound fx, quotes from the show where they dissed other characters, like one of those signature Vegeta quotes ("I've seen stronger houseplants!"). It would of course do very slightly less damage.
Or if you are like double the other guy's PL, they could throw in an auto-dodge for when you block, so that you get the opportunity to counter an enemy, kind of like when Buu and Gohan fight and Gohan just never gets hit; every swing Buu takes, Gohan appears behind him and melees him to hell. If you have a massive PL differential, maybe they could implement something like that, although it would be tough to gain PL on death PL-free servers. A guy with a fresh spawn PL tries to melee an 8mill PL supersaiyan, and the SSJ blocks; instead of a block he could be teleported behind the enemy, who will go into a confusion animation for like a split second where a normal melee attack would have made "impact."
Ehh. I like my first idea much better, but it's just a thought.