A standalone forum is not going to do well unless you already have a somewhat captive audience.
The general public wants a LOT more than just a chatter site. They can get that from their current social media apps that run natively on their phone.
Your site is going to have to bring together various features, and those features will not be consistent from site to site.
As you mentioned, a knowledge-base for certain niches are a given. As are true articles (not the poor imitations that some forum scripts push off a restyled post and and then calling it an article). Classifieds add-ons for niches that deal with hardware/equipment/parts that tends to get upgraded to newer versions and the older sold to new users looking for a deal.
A directory of resources. A dedicated review section if the niche relates to one that the niche items are something that others can experience/use (eating establishments, stores, vacation stays, this list can go on and on).
There are so many other ways that a site can push content that is not simple posts. And that is what a lot of users are looking for.
That is the mistake that certain paid script developers have fallen prey to, thinking that all they need to provide is a forum and the license holders that want more can then go to outside developers to get it. More and more are wanting those offerings from the first party developer because of one major reason... reliability of availability.
Over the 12 years of using various scripts I have seen 3rd party developers come and go in droves that were involved in every paid script. When your site becomes dependent upon a 3rd party developers add-ons and then they leave that environment (either to go to another script or getting out of it totally) you are frequently left in the lurch. Now you have to either find another 3rd party developer that offers a similar add-on AND can do imports from the other, find a 3rd party developer to create a bespoke add-on for you to replace it (this in itself has many of the same issues that any 3rd party add-on does) or hope that the developer that is leaving will transfer/give the code to another 3rd party developer to continue. Since many aspects of the features referenced above are not "small" ones, the giving away of the code is almost a laughable proposition. At the least that 3rd party developer will want to sell it to get whatever additional money they can out of it before leaving. Then you will frequently be buying the add-on you already purchased again, but this time from a different 3rd party developer who suffers the same vagaries as the others - the fact they may just get sick and tired of it and walk off.
Now, let's talk about the elephant in the room.... not using features of your native script that can enhance what you currently have.
As an example, I'll use XenForo and their custom user fields. A lot of folks are not that familiar with them, and if they are they tend to use them more for stuff dealing with registration. But they can do so much more in capturing data that can help users.
Now, the downfall of the above example? I can see so many ways that the custom user fields could be extended. Simply making them searchable would go a long way towards making content discovery easier for your users. Making them so that they can be used across different add-ons would help even more. As an example, the built in mirroring done by XF/XFMG does not even pay attention to these in a forum area and you have to recreate them and then enter the data again manually if you want it to show up. Just some basic short sightedness of developers on what is possible with just a little more work in the core or a position of "well, 3rd party developers can extend that if the license holders want it" attitude (which is the common one with XF from what I have seen). But these are things that if more people used the features and discovered what they are capable of would probably start asking for. Since they don't get used much, they are the ignored step-children.
And no, you can't throw everything into the kitchen sink... but some ideas can be used across different add-ons.
The big concern needs to be making it easier on your users to find what they need. That is still an issue with most scripts as discovery sucks amongst all of them.