I think it's brilliant that Scirra is working on user interface improvements, and I would love to see a feature that would allow me to store and edit particle effect properties, which could then quickly be applied to new particle systems that I create. These presets could even include sprite PNGs.
This would make it incredibly easy to reuse particle systems across projects and clone particle systems that have different sprites. Currently, the entire set of properties has to be reconfigured if I want two particle systems with the same behaviour but different sprites - e.g.: fire with smoke.
If these presets are stored as simple XML files in a particular folder, they could even be shared with other C2 users.
Additionally, support for child presets could allow for quick and easy management of particle effect variations, and having the ability to change presets at runtime would be invaluable for developers working on both desktop and mobile versions of their games; the Rate property (or whatever else is required) could simply be adjusted in a child preset and used for mobile.
Also, while I'm at it, I think a live preview of particle effects would be fantastic.
Keep up the good work, guys!