If you were Friar Lawrence, what problems would you foresee for Romeo and Juliet if they marry?

Any answer to a question like the one you ask is, of course, speculation.  That said, I'll try to at least base my answer on evidence from the play.

I suspect Friar Lawrence, in addition to the problem of in-laws mentioned above, might foresee two problems.

First, Romeo's sudden change of heart in act one of Romeo and Juliet is problematic.  One minute he's depressed over Rosaline, and the next he's elated over Juliet.  What guarantee exists that that won't happen again?

Second, and this is related to the first, is the impulsive nature of both Romeo and Juliet.  Impulsiveness does not necessarily lend itself to strong relationships.  With both of them being so impulsive, who knows what trouble they will get themselves into, together or separately?

For whatever it's worth, I suggest those are two problems Friar Lawrence, or anyone, might foresee in the marriage of Romeo and Juliet.