CExpr

https://voodoo-slide.blogspot.fr/2010/01/amplifying-c.html

which on large projects can easily reach several hundred megabytes of data

Pfft you wouldn't believe what variadic templates with auto variables do.

We left operator overloading mostly in the 90’s

Operator overloading is actually the only Sepples feature i like.
It makes a code bit cryptic, but it looks much nicer than typing function names into expression.
GCC has __attribute__((cleanup)) which is a scope destructor, replacing all the OOP i'll ever need.
http://echorand.me/site/notes/articles/c_cleanup/cleanup_attribute_c.html

>>3
I don't think projects like this will ever become more than prototypes. Whether it is a full language or just a small processor on top of C doesn't really matter for development because you still have to learn a completely new tool and at that point, you'd be better off if you just switched the language completely so you aren't held back by the C compatibility.