good discussion.
so the most successful page i've ever built was inherited from a major company that went bankrupt and was purchased out of bankruptcy by CBS. the core group sought a place to land that kept them out of the CBS reach- so i hastily threw together the site and tossed out the welcome mat. I share this as it's the only time/place i was able to build a forum first. usually the 'community' comes second after the 'anchor tenant' is established. this is most often a blog or some sort of informative page. i was able to take the crowd first and then introduce the publisher (publishing original content via community contributors). i also aggregate articles for users through permission from other publishers.
i just want to toss this out there for prosperity's sake: the wealthiest person i've ever personally known, who is very wealthy, once said to me "sell to the classes, live with the masses; sell to the masses, live with the classes". in bricks and mortar stores, this is absolute. however, in the world of the interwebz, it has been my experience it's quite the opposite.
i launched a cigar shop in the late nineties to support a bricks and mortar store(s) i was running at the time. I did this October 1. by closing of the year's books, which happened in mid January, online sales accounted for a full 1/3 of my sales. my local sales actually increased modestly from the prior years. this demonstrated to me the power of the internet. it wasn't easy, even then- i had to compete against several heavy hitters who were well established and 'destination locations' even at the time... (read: people sat in front of their computers and rattle-tapped THAT company name to seek the products they wanted, instead of searching for the product itself). there was only one way, by my reckoning, to counter this... instead of selling (metatag- huge at the time insofar as ranking was concerned) "handmade cigars", i instead listed "honduran, dominican, and nicaraguan handmade maduro robusto's", and hoping someone searched for one of those.... slicing a thin part out of the overarching 'handmade cigar' genre... it seemed to have worked. I managed to put together my own little ranking system for each of the 'headliner' cigar i focused on, and asked others opinions instead of being all decisive and confirmative about it... it wasn't long before i had a community- and wasn't long before i was playing with different forum software to support it... it would be a decade before i launched into SMF.
the point i'm trying to make is to actually be as distinct and narrow of focus as you can... attempt depth, not width. once you've got a few folks discussing items in your depths, then you can start to widen into complimentary items/discussions. it's been my observation, for instance, that folks who are into cigars are also into politics and often wines- so... take advantage of that. folks that are really into off-roading are also into BBQ'ing... there is an entire industry focused on these types of connections. they should be apparent to you if you are focused on something particular. it's up to you to interject discussions that allows growth/width- even at the risk of strategically 'hijacking' threads from time to time.
insofar as collecting audience, there is nothing better than expertise, if even only seemingly expert... for instance, find a popular website discussing your topic in a news/comment format, and you'll often find the same crowd in the comments... offer a comment like "that isn't what this guy says" and link to the article on your page... or conversely "dead on- this guy thinks so too", and same thing. do yourself a favor and reciprocate the link to the site your mining- offering links or even an advert free and unsolicited (until they tell you otherwise if that's the case). but all the same, from scratch? you gotta have something to offer- and that is usually information.
right now i have six sites that are direct interests to the primary site- pulling the community from the active one to the other, and selling them things i know they're interested in (because i've watched and listened to them for 17+years and 'internet' know most of them well).
it's hard work- don't let anyone tell you otherwise.... it takes commitment... tenacity... a few good breaks... it can be done, though. even in today's age. the opportunity remains unless this GDPR type stuff starts squeezing mom-n-pops or small operators out. my thoughts are that is precisely the intent of those initiatives, but that's just my opinion and we all know what those are like.
edited to add:
by the way... i don't understand the angst with WP... I use WP as the publisher as nothing anywhere else does as good a job from the users perspective, and i firmly believe SMF is the better of the forum engine softwares... they mate well- not in backend interaction, but can be styled to look the same to users, and that is the more important aspect. a WP landing with information and a patch to comment in the forum is the ticket, in my humble opinion, and crucial to developing community...