I propose to add support for start ..+ length expression, which is equivalent to start ..< start + length (e.g. 100..+5 is the same as 100..<105).
This is especially useful for slicing when start expression is quite long.
Also, I do not think start ..+ length needs a special separate range type, i.e. it can be just of type OpenEndRange with endExclusive set to start + length.