ceo: ultimately a catch-all, ideally has passable skills in most/all areas of building the product business. keeps the ship pointing in the right direction. has hand in basically everything.
designer: lots of experience with establishing product direction. very comfortable speaking to customers.
full-stack developer: has a solid grasp of every part of the app. manages repo(s). properly breaks projects down into bitesize pieces for the community to tackle. spends lots of time reviewing PRs.
content marketer: handles all content production. also has some full-stack dev experience to build mini-tools as part of the marketing cycle.
think you could build a $100m COSS company with just that team? $500m? $1b? where does that start falling apart? what/who is missing?