Here's my wish:

1. Get rid of the "Custom Occurrence" button.

2. Rename "Simple Occurrence" to "Occurrence" (what you got is no longer
"simple"!

3. Now I'm dreaming:
   Add to the month recurrence a "Plus X days" just before the
"Forever-for-until" button. This should take care of most multi-day events
where the first day is "pinned" to a particular day of the month/week. It
should  default to zero.

   Allow the automatic display of Holidays (multinational), where the user
can choose from a nice sized list. I'd include at least:

     Easter
     Hebrew New Year
     Chinese New Year
     Passover
     Christmas
     Thanksgiving
     Labor Day
     Veteran's Day
     .... and so forth. I'd include most religious holidays from 
           Christian, Jewish, Islam, Hindu, Buddist, most national Holidays 
           for several countries.

    And, finally, a recurrence based on a holiday, which allows several
other more minor holidays to be charted, "pinned" to the somewhat irregular
patterns of those holidays based on lunisolar events...

There's a good book on how to calculate these, "Calendrical Calculations",
but you might want first to discuss with the authors what it is they've
patented in relationship to date calcs.

Other cools things that could be done is the display of sunrise-sunset
times, and moonrise/set times. High/low tide for those near the ocean. And
how about moon phase, for those night paintball fanatics.

And, the last but not least cool thing, would be to allow different
calendars to be displayed. Like Chinese, Hebrew, Islam, etc. With native
month and etc. names. A lot of these would be interesting because the days
are split anywhere from noon to sunrise/set. And a few don't even reckon
time in hours/seconds, but in stuff like "moments"... oh well...