> E.g. the lagrange browser downloads images and displays then inline in the page
That's true, but it's based on opportunistic guesses, not semantics, which prevents many use-cases (including alt-text for accessibility of images). For example, we could do the same with video, but what about alternative soundtracks/captions? HTML has <video> <audio> and <caption>. Building a browser based on auto-guessing which link is related to which one and what the relationship is between those is... hazardous in my view.
> this allows the user to be in control over how these potentially abusable externalities should be handled, rather than ceding that control to the webmaster
I love that argument! But that was also the premise for the web, and i don't understand why we're not enforcing it there. CSS was supposed to permit user stylesheets, and JS empower users to script their interactions with the server. I would argue the problem with the web is it has moved away from declarative to imperative model where the server dictates your rendering. I can see how gemini addresses this by removing extensible declarative elements altogether, but i don't think that's the only way.
Personally, i think adding more semantic elements in HTML (eg. no-JS web components) would make client-side sheets more realistic while allowing the webmaster to propose a specific stylesheet for their recommended UI/UX. Removing the semantics is not gonna help empower clients, except when it comes to plaintext content. So, adding a new content-type over HTTP? Sure. Just throw in some CommonMark: it's well-specified and more or less complete (depending on your use-case). Adding a simpler protocol for delivering content? Why not, but it's not that easy to future-proof: HTTP is arguably more complex, but has some interesting properties such as content-type/encoding negociation or decoupling from transport security (for .onion/.i2p/DANE or any other future technology).
Just to be clear i like gemini very much and i've been wanting to support gemlog to my blog for a while. I just think there's room for some other technical foundations to explore the political promises of gemini.