Yep, I think most western countries will have a bunch of those weirdos within their borders very soon (if not already). Im just not sure how many of those foreigners join as a sort of “adventure tourist”. At least what I read you have many wannabee gangsters and rappers joining ISIS to simply live the gangster life with a bit of praying mixed in.
I doubt that these type of ISIS followers would pose a terrorist thread. They will be more vicious when they come back for sure, but they are probably not suicide attack material.
Alea iacta est ("The die has been cast")