The improvement never ends. Splitter can improve some fields of his game, he still has potential and I hope ambition.
But 3 things are :
Scoring Versatility: It's hard to improve after that age. He may learn some footworks, mid-range jumber and that's all. Post-up play? Not easy after that age.
Strength Conditioning: He can do it, he should gain some weight to his muscles.
Rebounding: It can not be learnt after 28 years old. Splitter will be a mediocre rebounder at his best.
CHAMPIONS SPURS - MVP DUNCAN