why not put in some really wacky items in the "fun mode", just for that mode? It's popular enough that it deserves more attention.
I would personally like to see "dummies" on the proving grounds. Dummies would be consumable ward-like objects that do not grant vision (they grant vision in an extremely short area, the enemy would literally have to be RIGHT on them for you to see them). The use of dummies? Katarina, Lee sin, and Jax lose a part of their playstyle because no wards are allowed. The use of dummies will allow them to have something to jump to, while being impractical for bush control, which was the key issue in removing them. Perhaps even make dummies not have stealth, so the enemies can just destroy it after it's served its purpose.
Also because champions are so randomly selected, sometimes there will be games that are just not fun for either party. For one because it is too easy, and for the other because it is impossible to win. Crowd control is a large factor in these games. Maybe make it less of a factor by putting in a consumable item (one time usable) that costs 1000g (a hefty price so that it can't be used very freely) to make yourself immune to all crowd controls for 4 seconds?
One last thing I would suggest would be making an item just for the proving grounds that energy users can buy that makes picking up sigils restore energy (around 50 energy).