string is rarely useful as a function argument

[a]

> Meh, I'd still prefer it be an array of UTF-8 code /points/ represented
> by an array of bytes (which are the UTF-8 code units).

By saying you want an array of code points you already define
representation. And if you want that there already is dchar[]. You probably
meant a range of code points represented by an array of code units. But
such a range can't have opIndex, since opIndex implies a constant time
operation.  If you want nth element of the range, you can use std.range.drop
or write your own nth() function.