I have already spoken about this before, but I have several new reasons of why would the implementation of this mechanic be a positive change for the community.
Of course, the requirements would be simple, at least a full team of six pokémon that are level 50 or above.
1. I strongly believe that casual players would use the unranked queue more often, as I have seen many rookie players wanting to battle without even having finished the elite four Kanto, but can't do it because they can't use the queue, therefore asking in the chats for a battle/challenging everyone in their screen. This would be a good way for novice players to learn more about pvp, since we all know fighting npcs is VERY different from pvp.
2. This would make guild events like themed battle ones way more easier to participate in (assuming you will be able to set all pokémon to level 50 if you want in challenged battles also) because the people who don't have the time to grind their pokémon to level 100, nor the money to pay for training services, will be able to participate easier, instead of having to go to showdown. Of course evs and moves will still take time and some money, but not as much as training all the way to lvl 100/spending rcs/buying rcs.
3. It will incentivate more players into pvp.
As I said before, the easier access with less grinding to an unranked pvp, will make those casual players more interested in pvp. I have seen the cases of many players that stopped playing because they were mainly interested in pvp, but they didn't have the time to get a full team, and going on with their life at the same time, therefore losing interest in it.
I must say again that all of this is for UNRANKED ladder, or for CHALLENGES. Not for ranked battles, as ranked is supposed to be for hard core pvpers, and I think it is perfect how it is.