Unfortunately, strings cross at least 3 different problems:
* charset encoding. Cases worth supporting include Ascii, Latin1, Xascii, WeirdLegacyStatefulEncoding, WhateverMyLocaleSaysExceptNotReally, UTF8, UTF16, UCS2, UTF32, and sloppy variants thereof. Note that not supporting sloppiness means it is impossible to access a lot of old data (for example, `git` committers and commit messages). Note that it is impossible to make a 1-to-1 mapping between sloppy UTF-8 and sloppy UTF-16, so if all strings have a single representation (unless it is some weird representation not yet mentioned), it is either impossible to support all strings encountered on non-Windows platforms, or impossible to support all strings encountered on non-Windows platforms. I am a strong proponent of APIs supporting multiple compile-time-known string representations, with transparent conversion where safe.
* ownership. Cases worth supporting: Value (but finite size), Refcounted (yes, this is important, the problem with std::string is that it was mutable), Tail or full Slice thereof, borrowed Zero-terminated or X(not) terminated, Literal (known statically allocated), and Alternating (the one that does SSO, switching between Value and Refcounted; IME it is important for Refcounted to efficiently support Literal). Note that there is no need for an immutable string to support Unique ownership. That's 8 different ownership policies, and if your Z/X implementation doesn't support recovering an optional owner, you also need to support at least one "maybe owned, maybe borrowed" (needed for things like efficient map insertion if the key might already exist; making the insert function a template does not suffice to handle all cases). It is important that, to the extent possible, all these (immutable) strings offer the same API, and can be converted implicitly where safe (exception: legacy code might make implicit conversion to V useful, despite being technically wrong).
(there should be some Mutable string-like thing but it need not provide the API, only push/pop off the end followed by conversion to R; consider particularly the implementation of comma-separated list stringification)
* domain meaning. The language itself should support at least Format strings for printf/scanf/strftime etc. (these "should" be separate types, but if relying on the C compiler to check them for you, don't actually have to be). Common library-supported additions include XML, JSON, and SQL strings, to make injection attacks impossible at the type level. Thinking of compilers (but not limited to them), there also needs to be dedicated types for "string representing a filepath" vs "string representing a file's contents" (the web's concept of "blob URL" is informative, but suffers from overloading the string type in the first place). Importantly, it must be easy to write literals of the appropriate type and convert explicitly as needed, so there should not be any file-related APIs that take strings.
(related, it's often useful to have the concept of "single-line string" and "word" (which, among other cases, makes it possible to ); the exact definition thereof depending on context. So it may be useful to be able to tag strings as "all characters (or, separately, the first character or the last character (though "last character" is far less useful)) are one of [abc...]"; reasonably granularity being: NUL, whitespace characters individually, other C0 controls, ASCII symbols individually, digit 0, digit 1, digits 2-7, digits 8-9, letters a-f, letters A-F, letters g-z, letters G-Z, DEL, C1 controls, other latin1, U+0100 through U+07FF, U+0800 through U+FFFF excluding surrogates, low surrogates, high surrogates, and U+10000 through U+10FFFF, and illegal values U+110000 and higher (maybe splitting 31-bit from 32-bit?) (several of these are impossible under certain encodings and strictness levels). Actually supporting all of this in the compiler proper is complicated and likely to be deferred, but thinking about it informs both language and library design. Particularly, consider "partial template casting")