Could you see a midfield 2 of Xavi and Frank existing without an enforcer? Their skill level will probably be ridiculous in MLS, but they don't have the legs they used to. That's my only worry about signing Lampard.
Glad you brought this up, as I've been thinking along those lines, as well, and I honestly can't think of a single MLS side that doesn't use an enforcer/destroyer. I believe a lot of guys are presuming Frank might play that role. Apparently, since it worked at another club, every attack minded or goal scoring b2b mf the world over can now just slide back to shield the defense.
I would personally have my doubts about that deployment.
It will also be interesting to see if Kreis utilizes much wide play, given our field dimensions and how he set up his prior squad. But that's another topic.