I started on a niche forum for a specific genre of horror. Because everyone was there for that exact niche, my first, very rough story got thoughtful comments from true enthusiasts. That encouragement was gold.
On a huge, general site, that same story would have been ignored. Sometimes, smaller and more specific is dramatically better. Find the forum or small website dedicated to the weird little thing you want to write about. You might have to dig, but the sense of belonging and relevant feedback is worth it.
Tutorials and onboarding for the platform itself are basic but vital. Does the site have a clear FAQ, video guides on how to format chapters, or tips on writing effective blurbs? A platform that assumes you already know how to navigate the world of online serialization is setting newcomers up for frustration.
The best ones hold your hand through the first post, demystifying the process of tags, summaries, and chapter formatting.
A private, invite-only collaboration space for co-writers or beta readers. With version history, comment threads on specific passages, and task assignment. Writing is often a team sport, but most platforms only cater to the lone author posting a final draft. Building tools for the behind-the-scenes process would be revolutionary.