I'd say you need to have a reason why your forum is better than what is already out there, and why a broad forum where any topic goes is better than a niche forum.
I'm at the opposite end of the spectrum from you. I run RC Naval Combat, a forum that is devoted to RC warships that shoot and sink each other. We have a custom design warship registry where you can show off your boat and list details about the scale (1:96, 1:144, etc), class (USS Iowa, HMS Revenge, etc), type (battleship, cruiser, destroyer, etc), local club, and more. We have a number of subforums for various aspects of the hobby.
That site has more activity and members than the warship combat subforum of a couple of major RC hobby forum sites. Those sites are much bigger, but look at us as the crazy people who shoot ball bearings at each other and sink each others ships. There can be helpful advice in the other subforums, but they just don't have the experience with fully submersing a ship and how we need our things set up differently in many cases.
Think about what exists out there already, and what you can do better. You're going to have this battle in many subject areas I would imagine. You need a value proposition about why people should come to your site for their discussions. It could be ease of use, mobile friendly, unlimited image uploads, a way of doing something way better than most everyone else is..but it's more than a build it and they will come proposition.