Primarily, the idea here that a maintainer may want to have IUSE="foo bar" and bar is recommended if foo is selected. Currently, the only way to recommend a use flag is IUSE="+baz" This doesn't work with the above mentioned example because IUSE="foo +bar" would recommend bar regardless of whether foo is selected or not.
For backwards compatibility, the IUSE + syntax can be maintained, but alternatively, it may be simpler, and make more sense to have something like:
IUSE="foo bar baz"
foo? ( bar )
So in this example:
is equivalent to IUSE="+baz"
The original inspiration for this idea was the following, app-emulation/wine works both with and without X. Most users who use X would most likely want the USE flags lcms and xcomposite, but setting them + in IUSE makes compiling wine with -X more complicated, as every flag that + just for X users would have to be unset for non-X users. The above system simplifies this by only setting + on lcms and xcomposite if X is set.
Alternatively, other names could be used, like DEFAULT_USE instead of RECOMMENDED_USE, but that is a side note to the primary point.
Any thoughts on the idea would be greatly appreciated, and thank you in advance for your time.