Like more distinction between the player, often we went in a portal at the same time and didn't know who we are, and when someone get hit display who win for a moment before restarting the level, and a way to know when a ball kill you and when you can pick it up and a way to tell how many ball you have
And also maybe make it clearer which portal lead to which, so for someone playing for the first time he don't have to try each portal to know were they lead.
Hope this is usefull! ^^