Well I guess they've left some things open to interpretation, as they should. As far as my opinion goes, after the rebuilding, Grunt became one of Wrex's top lieutenants and they worked together to make a more peaceful rise of a Krogan kingdom/empire. Other people may have other conclusions as to what came of him after the war, and that's cool too.
I prefer it that not everything is spelt out to us on the screen, so as to let us think a little.