I dont think that Groot and RR would have worked on their own in a movie. It may have done ok but it would not have been the level that GOTG was. As was pointed out by Wleakr they were part of a larger cast. So I dont think a stand alone with Howard would do well.
I dont like Howard but if they did it I would not mind seeing him as a parnter to someone else. He could work if he has someone to bounce things off of and him not being the main focus.