Cherry turned out very well. It's nice to have something with a sour-ish note. So I'd like to suggest some other sour fruits:
- orange / mandarin, which are some of my favorite fruits,
- lemon (the lemon cheesecake was also good),
- grapes, which may lead to a wine flavor
Out of those I'd probably like to try orange the most.
For coming summer perhaps water melon would be interesting.
As for some weirder suggestions:
* salmiakki liquorice (unfortunately this would have a rather limited market),