These are far from CSs as I only have a phone to work with right now but I saw the finished USAgent sheet and I wanted to chime in a bit.

Could do something interesting perhaps with Simpson America in Vietnam and US Agent, him not being what's expected.  I don't want him to be a caricature but he definitely won't have the stoic supportive patriotic attitude one would expect from a WW2 Veteran Captain America.

Steve Rogers Captain America 
Frozen in 1945, thawed in 1967 by Japanese whalers who found him, found out about Hiroshima and Nagasaki, Korea, Cold War, Vietnam, Civil Rights, JFK.  Returns to America via San Fran.  

His deal basically is that the America he believed in seems to be dead and he wants to bring it back.

Frank Simpson Captain America / Nuke
Idolized Captain America, wanted to be in the military, made it.  Wanted to be special forces, made it.  Wanted to be Captain America, that program was officially dead, couldn't make it.  Found a way.  Things got bad.

His deal is he is the new Captain America.  He fights for truth and justice and the American Way and if he he to do horrible things to bring that about so be it.  He got in on bootleg Super Soldier variations administered by an American Government agency that was under the sway of some corrupt dangerous men more interested in results than stability and more interested in conquest than in development.  He's a bad guy who thinks he is a good guy doing bad things for a good cause lead by bad men.