Here's something to play with: a Karplus Strong synth with harmonic frequency modulation. It's small and CPU friendly but you can get some pretty cool sounds out of it.

Have fun and let me know if you have any suggestions for how to improve it.

http://patchstorage.com/karplus-strong-fm-synth/

This is a pure Vanilla patch (that's a first coming from me!)

PS. If you search around for long enough, you might find a harmonic sweet spot (ie. try MIDI note 39, FM harmonic 7, FM amount 1).

Great sounds.
I would suggest making the decay automatically scaled to the MIDI note so that the high notes are a similar length to the low notes. You could try a similar scaling with the FM amount too.

Thanks for the feedback. Unfortunately, I can't think of any easy way of scaling as you suggest. The note length isn't determined by the envelope, but rather by the feedback decay factor. The filters seem to be a factor as well, ie. for notes in the 50s, the low pass drastically changes the duration of the note. So you'd have to write a formula that balances pitch, feedback and filter frequency, which sounds too complicated for my poor brain.

This is not a very consistent synthesizer--the charachter of the sound changes, sometimes drastically, from one pitch to another, and in some cases the harmonic modulation even changes the pitch completely (try midi-note 41, FM harmonic 6, FM amount 1.04, LOP ~ 7000!) On the other hand, I'm quite enjoying its unpradictable behaviour, especially considering how simple the build is. I think I'm going to replace the MIDI note selector with a line input for pitch, as it's probably better suited for sweeping sounds rather than melodies.

When I made one I just scaled the feedback amount to the pitch, maybe with a multiplier or a power, just did it by ear until it sounded a bit more consistent.
These kinds of sounds are also great if you use mutiples and detune them either slightly or by octaves.