Just a short list I can think of now:

- Short array syntax. Completely optional but would clean up the language quite a bit. This has been proposed several times and shot down each for no real good reason.

- Promote complex primitives into objects. With the proper interfaces it wouldn't break any old code. Once this is done, deprecate the myriad of str* and array* functions in favor of the object methods. This also cleans up the "what order do i pass in the args" problem that so many people cite.

- Named parameters. Gets rid of passing in default params or an array blob for functions/methods that take lots of optional params. It also opens up the way for DSLs.

- Replace PEAR with something better. The quality of PEAR modules is really low and PEAR always seems to be broken one way or another.

- Add a list and hash type. PHP's array is very slow for some cases, and sometimes you need to enforce the datatype you're working with.

- Use exceptions for errors. Could be a runtime flag to prevent breaking things. It's incredibly easy to write bad code because PHP make it easy to ignore problems. Things like functions that connect to the db or trying to retrieve data from a stream don't throw exceptions on error.