Compared to what is out there right now, C2/C3's particle systems are looking pretty dated.
Few extra features for particles that would make them more universally useful and without the overhead of sprite objects used in their place.
Rotation - smoke, fire, projectile splash effects etc.
Set angle - set angle at spawn of a particle to direction, useful for sparks, bullet casings, leaves etc.
Lifetime events - a trigger event for when a particle dies - emit particles for example. useful for fireworks, flames breaking off into smaller embers, water splashes etc. Could obv enable "less experienced" devs to run their performance into the ground if abused, design a limit or warning message?
X/Y coord of individual particles to use alongside lifetime event listed above (spawning in new particles.
Frames - additional frames in the particle editor menu aren't used for animation, but are picked at random for spawn. Instead of making 3-4 different flame particles for a nice varied looking flame, you can put the 4 different types into the frames, and the particle object just picks one at random on spawn.
Hue-shift - enable a particle over its lifetime to change its hue/sat/whatever. Useful in smoke and fire for example with gradual changes in colour - fire red to orange/yellow
Random fade out - or something to that effect. "Objects fading out at once looks unnatural"
Could well be a few more suggestions, but I think what i've listed above would really up the visual quality of games - nevermind a few would be "optimizations" over using a mass of sprite objects for the same purposes.
A new mode for the Particles plugin that spawns objects instead of drawing its own particles shipped with C3 r121. Spawning Sprites gives you full control over each individual particle. You can even spawn other Particles objects for crazy effects!