While the original TeX engine only get bug fixes these days, that's not really relevant for LaTeX since LaTeX doesn't even support that engine anymore. While pdfTeX development is rather slow too, it does still see feature development. That being said, for modern documents LuaTeX is really the only engine worth looking at and it is quite actively developed. Of course, it doesn't see much adaption but that problem will also appear for any new system.

That brings up the other big problem with contemporary TeX. LuaTeX has handfuls of subtle incompatibilities with pdfTeX and XeTeX (and a weird way of handling character setting at the low level where it maps characters to their index in the font file rather than unicode code points). It's also noticeably slower than XeTeX which is slower than pdfTeX. It would be nice if it were true that pdfTeX ⊆ XeTeX ⊆ luaTeX, but unfortunately, it's not.

> but no macro language

Any reason for that? The rest of your description looks very attractive, but why would I prefer a language without macros? I need conditional compilation, setting variables, parametrized notations, etc. This is extremely useful when preparing a scientific document. If anything, I don't really care about the document preparation language itself, but it must have macros to be useful!

To clarify, there would still be programmability, just not via macro expansion. Instead of macros there would be first-class variables instead of TeX's mix of registers and macros to emulate that. Instead of having to deal with the timing of macro expansions and all the pitfalls therein, an embedded scripting language would be available to handle complex settings (I'm leaning towards Python at the moment, but I'm far from having to make a decision on that topic). First-class flow control will also be available (right now, things like \foreach and \ifXXX are handled via macro expansion which causes a lot of pain beyond simple use cases).

OK, that's reassuring! What you describe sounds pretty much like a "macro language" to me (i.e., a separate language to describe your document). But this is just a question of terminology, somewhat arbitrary. Being able to generate the text of your document is a very powerful tool and, according to what you say, it will be even more powerful.