3 things Marvin needs to be a beast:
1. Avoid being the clumsiest driver on the planet
2. Grow some cajones and take charge more offensively...be a leader
3. Stay involved in the game even if your offense isn't there
*Please note: these 3 things need to happen on a contract year for Marvin. Sadly, none of these 3 things are within reach.
Either way, the overall skillset is there. He just doesn't have the mental drive with the game of basketball to put it all together. And if he hasn't at this point, he never will. I think he's satisfied just coasting through his NBA career, not doing anything significant. It fits his laid back personality.