There has been a big movement of top Americans back to MLS the last few years, so there aren't as many top Americans still overseas. There's Geoff Cameron at Stoke, but he seems happy there. Yedlin is at Tottenham, but he just moved. Likewise Zelalem, who is developing well at Arsenal, but he's not likely to leave. There are more players in the 2nd division, but those guys are less likely to be upgrades over MLS starters.
There are quite a few Americans playing in Germany who would be great targets. Guys like Fabian Johnson and John Brooks to name just two. But, many of these guys grew up in Germany, so they are less likely to leave. Alejandro Bedoya, who plays in France, would be a good impact target. I think there are now a bunch of guys in Mexico who are starting to emerge, like Garza and Alvarado.
Then there is Aron Johannsson in the Eriedivisie, who has been discussed a bunch. He is a good target to come to MLS, but securing his rights will be tricky for reasons that apply to many of Americans playing abroad. Most of the good ones are on the "Allocation List", which means you need to secure the right to sign the player from the top team on the list before you can sign them. That costs extra and can be tough to negotiate.