Currently there are three predefined String types in package Standard (String, Wide_String, and Wide_Wide_String) and a collection of packages that support Unbounded, Bounded, and Fixed versions of these, plus some support for converting to and from UTF-8 and UTF-16 representations, etc. The community is looking for something analogous to a "universal" string type which can be used in almost all current contexts where strings are expected, fully supports Unicode, and which is as easy to use as a numeric type.
Nico Roche of AdaCore has done a thorough study of various issues that pertain to improving strings in Ada. A copy of this document is in the Better Strings Study Group Google-drive Folder, included below.