My prediction is that you will be able to do co-op online mission with your custom character to get material for synthesis.
this is a level 5 game so you gotta be combining stuff in some way and online co-op monster hunts would give people a reason to invest in there custom character.
many, including myself, would have just made the custom character and used them until a better story character joined the party (if you can remove the custom character from the party that is).
if level 5 were to give the CC an important role like being the only character you could use online, they would be come much valuable to the team.
also this system would allow for a lot of DLC in the form of new armor, weapons, and missions