I'd like to run (about one session each) to see how ideas work before messing too much with the SD system:

* Dying Earth: for the "object to failing a roll IC and re-roll" mechanic.  Skills are rated as in most games (scale is 1-6, I think), but all resolutions are just a single d6.  4+ succeeds, 3- fails.  If you fail (or your opponent succeeds), you spend a point of your rating, object in-character and reroll.  Points refresh when you debauch properly.  1s and 6s are more expensive to reroll.  This makes highly skilled characters approachable, while still powerful.

* Adventure!: for the DramaticEditing: every player has a pool of Inspiration points, which power the setting's superpowers and superscience (think TheShadow, IndianaJones, TheRocketeer, PulpFiction).  They can also be used to briefly usurp the role of GM: 1 point for minor offscreen effects, 2 for minor onscreen, 3 or 4 for more major effects, +1 for a plot ramification (e.g. spotting a clue for the current mystery), -1 for a plot complication for that character.  I've heard it works well to simulate pulp-style luck... Ariel's been trying something similar with her Seeker character, but it seems to be failing in a situation where most of the party doesn't have it.

* Trinity: for how non-adepts in a Bostonish setting would feel.  Maybe for the free-form psi system, which bears great similarity to the intuitive magic setting component (though not to the system we're using now).  Plot idea: investigation of a crime in Rio.  Norca interference, the local Aeon chapter isn't entirely trustworthy.

* Mage: only maybe, to see how the magic system plays: if we do go pool-based or away from spell lists, we need something good for Intuitive and for Disciplined magic.  I'm less convinced than I once was that a single system for both is a good idea.  Mage might make a good fit for the Disciplined system, the Intuitive system, or a unified system: it's very flexible.
Plot idea: Clue!/Ten Little Indians.  Several cabals of magicians (base characters + 50 XP or so?) are in a chantry for a meeting when something goes wrong; bodies start showing up.  Target: no more than 5 characters to a cabal, so potentially two PC groups.  Both should be visiting, not fully local, but all have been here before individually.  People keep dropping dead, which might even include the PCs if they're dumb enough to walk around alone.

* Nobilis: what can be done without dice?  Plot idea: New familia; your Imperator has been accused of a crime.  He establishes a chancel to clear his name and deal with the politics of the War while he gets back to the front lines.  The PCs are that chancel.  There's a neighboring chancel which is neutral but could have witnessed the crime, and a familia of inquisitors.

* Exalted: to see how SD might convert over, and to try some ideas for party creation I've had: you generate an Exalt, the GM hands you back a mortal character sheet, and you evolve into that exalt over the first month or so of play, meeting the others as appropriate.  Also, I'd like to see how Sorcerers play for comparison to the Disciplined Magic system: rare spells, but you probably have a few minor demons/elementals/olliphants you can order around.
http://shadowdawn.evenmere.org/exalt/NewGameNotes has my notes on that new game — y'all are invited.

* GURPS 100-pt fantasy, to see how the system works at its original target level.

* Maybe Feng Shui or Unknown Armies, for a bunch of reasons to fill in details above.

* Maybe Earthdawn or Shadowrun at various power levels, to see what things feel like in the original games.

If you're particularly interested in seeing how one of these works, let me know!  Heck, run it yourself!