I don't think there is a need to offer some type of incentive. The majority of modifications created for other software are done either because the author required it for his/her own site and decided to share it with the community or they're offering a 'lite' version with hopes of attracting potential clients towards a paid version. Of course there are people who just enjoy developing code and they release their work just for the social aspect, community involvement or whatever

I'm sure you'll see a modification community here in time with names nobody has ever heard of. Their ideas will be fresh and every bit as good. It just takes time. All forum software communities have their 'coders', XenForo won't be any different.