What do you think presently I think it would be pretty cool to be in control of a military

Still not sure about what the US would be in JC5. Would be somewhat controversial if they set the entirety of the US as the enemy, and the player launches an invasion on US soil.

It would have to be on another fictional island

If it is not set in the United States

Then Avalanche would come under fire for basically deceptive advertising.

If that Agency logo around Washington D.C. is not some Agency military base

Then the Danger Rising DLC will basically have been for nothing.

Do you think all Americans including some of those patriots will be happy about the game telling you to invade american soil?

It's not advertising, its teasing. So yeah, maybe some people would think the teaser at the end of the Danger Rising DLC was for nothing, but that's how the JC3 DLC was, too. Whatever happened to the "global change project"? At most, it became project illapa. But most likely the devs forgot about it. Same might happen for Danger Rising, or maybe the Agency and the US will be viewed as separate factions so the story makes sense without attacking the US.