Edit -- ignore my ignorance, I misunderstood the intentions.
Seems like a backwards move, IMO. Move from full-featured software to something you have no real control over. I suppose the most logical choice would be to move to a subreddit, but nothing really comes close to XenForo in terms of being both functional and full of features. If you're set on moving to something different, why not try something like Discourse or Flarum? You're not likely to find many favourable opinions on alternatives here, all things considered.