As a Roblox developer, it is currently too hard to do fast sphere casts in Roblox.
If Roblox is able to address this issue, it would improve my development experience because it would allow for much, much more complicated interactions in games. Unity has a sphere cast feature, and its effectively a raycast with a spherical radius all the way around it. (You can think of it like a sphere at each start and end point with a cylinder in between).
Sphere casting as a feature would allow for more projectile types, and much much more CPU efficient custom explosions. Sphere casting in-engine has the added benefit of not requiring the use of physical part objects and thus would be significantly faster than current methods.
Additionally games like Super Smash Bros use something similar sphere casting for combat, and if I’m not mistaken do in some cases use sphere casting. Sphere casting would be very useful when you need “big raycasts.”
Also, @Elttob pointed out below that Boxcasting would be a similarly appreciated feature. I think that raycasts, boxcasts, and spherecasts are all very important, and it is extremely important we have efficient means to perform these types of things.
And, ideally, raycasting, boxcasting, and spherecasting should be as fast as they possibly can be, as they are fundamental to some of the most core mechanics in a lot of non-Roblox games. All around I think these sorts of casts deserve focus.