So I've given this thread some thought, and I noticed you said you wanted to make an accessory upgrade for each of the four pillars. In my personal opinion, the best one to use on boots would probably be Vortex instead of solar. I'm not taking the classes (melee, magic, ranger, summoner) into consideration, but rather what the pillars and enemies of these events actually do. The Vortex zone in the celestial events seems largely centered around lightning and gravity, which seems very appropriate for a speed-related accessory in my opinion. Now, the trail of damaging sparks? I don't think it's really necessary, but I guess if you really wanted it, the Vortex Boots could leave sparks of lightning... or something.
Now, the other reason I wanted to change the boots to Vortex was because I think Solar is much more fitting as a Fire Gauntlet upgrade. It would inflict Daybroken instead of On Fire, and maybe have +15% damage and speed instead of +10?
Now, Nebula and Stardust... Nebula could be an upgrade to the Celestial Cuffs, which causes enemies to drop mana stars more often and provides an even bigger star pickup range (just an idea). Stardust, a papyrus scarab upgrade? I can't really think of anything better.