I think Mo is going to hit. I wouldn’t be surprised to see a guy hitting .280-.300 with 15-18 homers as his best case scenario. Maybe a wRC guy of 125. I am not suggesting next year. But your question is best case scenerio. I took it to mean as he plays a while in the league. Now, the issue is, can he catcher. I think for that to happen, the best case scenario this year would be the Cubs get another bat who can DH and play outfield. Leave Mo at Iowa this season and let him catch every day. Maybe (again, you asked best case scenario) he can become an average defensive catcher. An average catcher with his hitting ability is an all star catcher. That is the ultimate best case scenario.
I do think he will DH this year and do well. I am talking ROY contention, well. I am fine with him on the team. But in doing so, IMO, they are deciding his position will be DH for his career.