Better plot idea

I think one of the ideas that got tossed around was Rambo taking on some out of control private military crews like the Blackwater and what not. Lot of potential in a story like that.

Plot Twist: They brainwash him and the movie ends with Rambo being a suicide bomber in an Old Country Buffet

Given he basically helped them fight the Russians in Rambo III that might be a bit hypocritical.

and the evil aspects of Islam

You mean Islam just in general