Changing the scheme doesn’t really make any difference if it’s still just HTTP underneath. The scheme is just for indicating a protocol. So what’s the different protocol you’d actually propose?

The disadvantage is, Reddit and other platforms will never add support for [fediverse hyperlink](fediverse://example.com/post/1337) Markdown syntax, or even start blocking it (they can already block known Fediverse domains but there would be backlash if they did).

Absolutely not. It should run on HTTP, as a website. Unless you want to build a client which would be somehow fundamentally different from a web browser somehow (note: Lagrange and Gopher Browser are just browsers), which would somehow be able to display data from every use of ActivityPub / “the fediverse” in a different context from a web browser, then no. What we need to build is website software more in line with kbin / mbin, collecting together all the different information of the fediverse into one interface.