I have seen many forum owners handle upgrades by restricting what regular members can do. Premium members have additional permissions such as avatars, access to special rooms, private messaging, custom titles, and more.
I also thought it would be good to display ads to free members and remove them for paying members; though I am not sure how much this will tempt people to upgrade seeing as adblockers are becoming more popular.
I am reluctant to use an add on to give more features to members as I know that long term, most add ons lose support from their developed. So I'd rather define permissions and options through the default version of XenForo.
Would love to hear what you guys are offering your premium members