Celestial shell: provides a speed boost, regeneration, infinite swimming and underwater breathing (only niche thing about it), armor, attack speed, damage, bonus to all of these at night... probably a few other things that I forgot...
Terraria Community Forums
Dapling
Dapling
Lavaspark: can walk on liquids (niche), temporary immunity to lava (NICHE), can walk on burning blocks (NIIIIIIICHE!), speed boosted on slippery blocks (NIIIIIIIIICHCHHHHEEEEE!), extremely short flight, large speed boost.
Dapling
See? The only thing niche about the celestial shell is swimming, and the other things about it are powerful enough to be their own accessory. There is literally no reason for lavaspark not to exist, if you're worried about it being available too early just slap some random hardmode material into it's recipe...
Dapling
Lavaspark would be just as powerful as the ankh shield: strong, but rare enough that it's often not even worth going after. By the time you get it, you have other options which are much easier to get.