- define the evaluation order for function parameters, e.g. f(a(), b()) [or is that well defined in modern C++?]

- allow for named arguments. E.g. let's say for the definition f(int a = 12, int b = 42), one might call f(b: 1337) or f(12, 1337). Not allowing mixing of named & positional is probably a good idea.

- take a look at static verification and remove language features that make static verification more difficult and think about how you could replace them (or remove; but e.g. function pointers & similar stuff fall in that category and are probably to powerful to be sacrificed this way)

//Edit: as others have said, try to whack as much undefined behaviour as possible (and in case you can't, don't accept the input program).