In no particular order:
Multiple inheritance is messy, complicated, and rarely useful. Personally I would favor singular inheritance for classes and multiple inheritance for interfaces, which brings me to my next point.
Abstract Base Classes, Pure Virtual Functions
Instead of ABC’s and pure virtual functions, it would be nice to have real interfaces!
Template Meta-Programming (a.k.a. Black Magic)
This is the kind of thing that makes programmers cry at night. The code you write for compile-time should look like the code you would write for run-time! Take a look at the D programming language if you want to see this done right.
I see no good reason for having fixed-width data types hidden away in libraries other than for backwards compatibility.
Const and Virtual
const should be default for variables as it encourages good practice. It would be better to have a mutable keyword.
virtual should be default for member functions as it is usually wanted, and in some cases quite disasterous to forget (e.g. for a destructor.)
Implicit Class Members
I would personally not provide implicit members. Instead I would only offer them explitictly (i.e. = default.)
There is a lot I don’t like about C++’s streams, I won’t rant about it all here but it could certainly use a modern redesign.
C Preprocessor, Namespaces
The cpp is garbage in general, but #include directives and namespaces would be better off replaced with a proper module system.