Depends on your goals but I think a minimal and useful browser with completely new APIs wouldn't be so hard to do if you leverage the huge amount of existing open source libraries. The hardest part would be getting people to actually use it.

Make it markdown based. It'll be like the web once was... Just documents linking to other documents, with images and videos. We just pretend web 2.0 never happened. Everybody can write markdown so we don't even need web2.0.

You might wanna look into Gopher and Gemini protocols, as they seem to be pretty much exactly that

Gemini is probably not exactly what GP wished for, but it has something resembling a critical mass of users and while I do not think the text format (gemtext, i.e. gmi) is perfect I find it good enough for what it does.

I even use gemtext now and then offline just as an even simpler markdown. Since it has so few features it is trivial to convert gmi to markdown or to any other format without losing anything. It works as a lowest common markup language for when something that minimal is enough.

Can we please not switch to a format which is simultaneously more complex and less capable than HTML?