1. the board should have some unique theme or some technical feature that makes it stand out sushichan for example used vichan but has original take on it, lainchan of course is all cyberpunk, leftypol is commie themed, this board is dream centric
2. You need a community - that can take a long time to build from scratch, so having friends and aquaintances who will be the initial posters and moderators is a big advantage, so being involved in a pre-existing community on reddit, or discord or something is useful, the board can serve as a bunker or backup for groups that get censored, having a dedicated IRC or Matrix room could also help.
2. the board has to run good, you need to show off some basic html js skills and don't just use the default from vichan, people have to trust you and won't take the time to post if it seems like it's a honeypot or that the site will crash at any moment, you should also consider user security by using a decent hosting provider and maybe creating tor or i2p hidden service
4 - optional: mobile support really helps, lainchan or sushi can be accessed by mobile apps, most humans use mobile browsers, site should be at least mobile useable