I think the main focus of Invision should be on Forums, I mean that is why we are all here in the first place isn't it? The extras are always nice Clubs, Pages, Ecommerce etc. These are nice and all for extras, but the root of the whole experience is forums. There are a lot of add-ons that could be worked into the forum root like @Adriano Faria posted above. There are some quality-of-life experiences that I feel are missing from the forum experience that should be built in from the beginning.
Tag editor (it would be nice to have a page built into the ACP where you can view all the tags added, modify, and delete these).
Updates to email such as the Drip campaign add-on. There should be a built-in system that notices if a user hasn't visited in a while and sends them an email showing them what they have been missing. Sometimes people just get busy and forget to come this, this would help pull back users to your forum.
Widgets! We all love widgets; it would be nice to see more added. Invision has a genuinely nice drop in place system for these that the competitors can't touch. Expand on these
Keep the monthly updates coming. One of my favorite things about switching back over. I love seeing new updates coming. (Maybe I am a nerd 🤓) A. It makes me feel like the devs are working for your money and you're getting something of value. B. It's nice having things patched in a timely manner and not having to wait months or years. C. I love when you guys introduce new features, again like A. This makes you feel like you are getting something of value and the software is not going stagnant.
These are just a couple that come to the top of my head that can really add value to the whole forum experience and are standard built into the competitors forum software. Owners' engagement on this forum is also a plus. It's nice to see post from @Jordan Miller since switching back over here he brings life into the forums. Anyways, these are some of my suggestions.