Lets not forget monetisation, being able to monetise videos uploaded to your site using Adsense (I assume that's what YouTube uses) could be a big benefit. Although you'd need a lot of views for that to work.
That's where an Amazon based back end may be better for larger sites or forums with sensitive content. You could enable custom pre-roll ads to play in house sponsored ads before the content, I'm sure you could make more money than AdSense that way, especially if you offer the removal of all video ads as a user upgrade benefit.
Even without that though, just getting easy video into posts via YouTube would be such a big feature and be suitable for the majority of sites who just want the upload capability.
A decent newsletter digest that scrapes the most popular content of the week from all sections of the site could be good.
Most popular posts of the week by views/replies
Most viewed images/galleries that week/month
New social groups
New blog posts
Nicely designed, actually including thumbnails and a few images to make it pretty, readable and encourage the users to click and come back and see what's new regularly.
Discourse has a pretty decent one built in if you want to check it out.
If you wanted a big project you could work on a new Social Groups add on.
Nobita's is the best I've used, and there's a huge amount I like about it, and parts he's implemented really well, but it still seems lacking in many areas and progress has been slow with a few long standing bugs.
I like that there's a wall for quick posting and chit chat, yet also access to full fat subforums and that it ties into both major galleries and has an events system built in. This is another reason why I'd like a calendar in the core - so that any add on that needs events could connect to it as I effectively have two independent events systems on my site now. XenAtendo for 'General' events, that creates a thread in a normal node, then 'Social Groups' events that only appear to members in that particular social group.
The SG concept seems to be very popular with the new IPS, but I reckon it can be done better. This feels like a huge project though if it's done right.
If it needs to be priced accordingly and if the quality is up there though, then I'd pay for it.
Perhaps two competing high quality social groups add ons would drive progress faster? I'm not sure.
+1 for a good Social Groups addon. I reached out to @Xon about this a while back, as I had a hard time wading through the ones that are currently available...unresolved bugs, not updated in a long time, need additional features, has a lot of bloat, etc...
It would seem to me that having a SG addon that is segmented for various uses would be good. Like a base package, and then enhancement packages for various features.
We're using this add-on to archive/delete threads automatically and we have another add-on that moves threads to an archive based on prefixes. For example, if someone has a thread about hosting an event and the event is over, they can change the prefix to 'finished' and it's automatically deleted or moved to an archive. It would be nice to have one all-in-one add-on that would archive/delete/lock threads based on prefixes, age or other criteria.
For custom thread fields, it would be useful to be able to assign multiple sets of fields to a single forum. For example, a classifieds forum could have separate sets of fields for buying an item, trading an item or listing an item for sale. (We also just really need a classifieds add-on!)
Our members want a way to organize their watched content into folders like they used to be able to do on VB. We had an add-on developed, but it doesn't work well and won't be available in XF2 as the developer has left.
Another add-on we miss from VB was Easy Forms. XF has Simple Forms, but the developer left, so I doubt it will be available in XF2 unless someone else wants to take it on.
I was dreaming for years if each user could customize his own "what's new" overview page as intuitive drag&drop modules. Personalizing/filter content and discovery content in usability is a mega trend.
Also a 2 or more columns grid would be awesome to use the space on big screens and give as much as possible different modules to the user if needed. I wish just more options for customize widget positions than only single blog post style (multiple widgets per row possible instead of only one). Probably resizable widget modules by each user for himself.
Then other addon devs could build more modules for it and their own addons. So you provide a "platform" (probably sort of an ecosystem) which will be demanded
I mean, the main thing I'd take from Facebook is simplicity. People get lazier and more impatient. They don't want features, they want the basics and they want them available simply, without being overwhelmed or having to learn lots of new stuff. That being said, forums aren't Facebook. Gotta find the right balance to stay relevant and remain as forums.
Should, ideally, be focusing on removing features or making them less visible.
A few misc. changes in XF make a perfect bridge here.
XF2 looks sleek and keeps common functionality fairly visible. I like that a lot.