Some would argue that with the Marine Corps we already have our own Foreign Legion...

Seriously, though, it might benefit the Marines to look at the old Lodge Act, which opened service to non-nationals fleeing Communist governments in the 1950s. Something similar might prove useful for them.

As for an actual Legion...I can see way too many political problems for the US in setting up such a force. "Foreign Killers Hired by US Government" "CIA Mercenaries Given US Uniforms" Take your pick.

I would also hazard a guess that the French are being premature in phasing out the Legion. All professionals are not created equal, and the Legion brings certain intangibles to the table that the regular French army will be hard-pressed to meet, in addition to being able to draw from a larger recruiting pool.