As a Roblox developer, it is currently hard to design your game around allowing for local multiplayer and it is impossible to have local multiplayer with independent characters. I have designed one game which had local multiplayer as a possibility but it was very limited. Local multiplayer on Xbox with a split-screen functionality would be a step in the right direction with allowing friends and siblings to play Roblox together on the same device. The Xbox is also the only platform that Roblox is on right now which guarantees enough power to be able to handle the job of rendering two separate images. I also only see split-screen working on console compared to pc or mobile.
If Roblox were to add local multiplayer support for Xbox(and whatever consoles Roblox may be on later this year), I believe that would greatly increase the joy some players get out of playing Roblox. This would also help developers to make their game more entertaining for their audiences. Of course, some small details such as the ability to do vertical, horizontal, quad splits or no multiplayer screen splitting at all would have to be implemented so developers have more control over their games as well as some way to detect if the game is currently split and how. These would all be necessary to make split-screen multiplayer as smooth and fun as possible. Of course, I see this being a really big challenge for Roblox if they decide to do it, but I believe it would definitely be worth it.