THe goal ref was a absolute failure in the test they did this year in the Europa League.
The number of refs is fine, the game only needs two things, first, game suspension for obvious divers. The reviewing of card sanctions after the game already exists in most leagues. Its pretty bad though.
Second. Phantom goals should be seeing in video replay or they should put a chip in the ball that tells the ref when the ball has gone all the way in the goal so the game is not stopped.
Offsides and handballs should be left alone, they are part of the game. I dont know for the challenge system, I dont think it could work. 1 call for each team is not enough, they would never know when to use it, specially because the players and coached are wrong most of the time themselves about what happend in a play. There are so many calls that are 50/50 really.