How much would I be looking at?
I don't really see a big of a issue with that, you build a simple query $where clause, then pass it trow a hook for external plugins to add new clauses and run the query.

You log all users activity (well...) and the query will decide which items to show (filter by UID, FID, etc). It actually sounds like a good idea.

Sounds rather like a nice idea.
If you're willing to give it a whirl, go for it Happy I haven't got time to be taking on another project just now anyway.
As long as I can release it for free, yes. But I also lack of time right now so I will be working quietly on this.
I'm sure that would be fine Happy There have been a few requests for a similar plugin on the MyBB community anyway.
Yeah euan that's what I wanted the follower plugin to do basically (cba to quote you).

Not sure I'm up for spending that much for these features. I think the move is inevitable, but won't happen until after Wrestlemania. The conversion process could be a pain, as no one has confirmed likes/passwords remaining, but I was told it was possible. I've already sent a copy of our database to a well known member and importer there though, so I'll soon find out.
Will be interested to know the answer, keep me informed please Happy
Really, I don't really think you will need to pay for all those features (except the profile page). I would do it if I had the time (I actually have some code for the threaded PM).
If I wanted them done timely with quality I would. It's hard to find someone with the time and motivation to do something like this for free to the best of their ability.
Very sad if you do move away from MyBB, but I can understand why. I'm mostly with MyBB for the insane flexibility and xThreads – if those aren't draws for you and money isn't an issue, it's easy to understand why you might want to leave.

Rather than paying $300 for a custom plugin I would really like to see it released for like $20-30 ... surely at least ten people would want it.
Which plugin are you talking about in particular brad? Personally I'm hesitant to charge for plugins due to my unpredictable schedule sometimes getting in the way of update releases.