Yea, 4-4-2 seems more logic. I guess Bellerin RB and Vidal/Roberto RM, Neymar LM, Suarez CF and Messi SS.
I don't see any other position for Messi other than SS, he can't play in midfield.
No way he is going to play 3-4-3, he is special for his defense. Also, he said that defense is priority.