Well its interesting to note that its the only great spirit with out range 3 (it has range 1), so for one thing it would be nice if they gave it some range (even range 2 would suffice but for consistency sake 3 makes the most sense). Its lack of range was justified when its effect was the most powerful of all the great spirits, but now that its effect is the weakest, having such terrible range as well is pretty odd.
Also since its now more comparable to salamander then it is to Shade, and salamander is powered not just by his effect but his 30 attack, giving will-o-wisp 20 attack instead of 10 might not be a bad idea. Not strong enough to 1 shot most things like salamander but atleast able to ding the armor of 10 defense units, and maybe if your lucky take out a haste soldier every now and again.
I also wouldn't mind having it JUST give +1sp and have those stat boosts. +1 sp isn't as potentially good as shades -2 sp, and 2 agi is still slower then shades 3 agi, but the +1 is more consistent than shades -2 which might fizzle and he's be a slightly more viable offensive threat if he lives long enough to get his attack off.
Artforts suggestion would work great too, though that would make him even more odd-ball in comparison to the other great spirits, atleast it would make him useful again. The terrible range, attack, and +10 hp boost still isn't comparable to the range, open skills, and attack of the other great spirits, but the +1sp would compensate so that at least he's the "cheapest" of all the great spirits since he effectively costs you nothing to play.
The bunnies of Lavato have special abilities, like 'Action Skill: Make Carrot Disappear.'