The 6+ feels very nice in my hands with its case on. However, I have android phones that are heavier when inside a case, like this Note Edge, and they feel absolutely fine too. The 6+'s batterybis already great though IMO. If you need more there's Mophie.
Bottom line No one is going to be satisfied no matter what. People will complain if the phone is too thick. People will complain if the phone is too thin. People will complain on just about anything. The iPhone 6 is awesome just the way it is. If you want a thicker iPhone go back to the 5s.