You know my name? It's Mysterious, so I can't show anything here.
I do get your point, Something that would help would be if devs created a closed APK with some changes regarding nerfs and meta changing in Arena. They could choose some players to test those changes and report back to devs, so we can make mystics great again, their nerf was even worse than Healers.
Awfully convenient eh, claiming neutrality while keeping your lineup unrevealed?
Nothing was changed to mystics; only DEF was buffed to mitigate MAG damage, which indirectly hurt the over-dominant mystics back in the day. DEF itself needs to be reworked(to mitigate by percent instead of flat-reduction) to resolve:
1. Berserker's over-dominace
2. CRIT's interaction with DEF
3. Mystic's splash damage to actually do damage
I get the idea of your suggestion but I worry for the lack of motivation to commit resources for a PTR version so instead, I suggest a online player-vs-player arena mode where players could experiment with any sort of lineup with the ideal faith, LB-ed to level 40 and equipped with equipment of the highest tier. Players are then pit against one another to test the effectiveness of their experimental lineups. To up the stakes for a competitive version of this(where rewards are earned), points are earned for victories and lost for losses but on top of that, a pick/ban phase similar to that used in dota could be introduced. This way, players get to truly experiment for their lineups and define the meta by identifying the strongest heroes without committing their monetary resources beforehand.
I think you don't really remember what happened with mystics, but let's remind ourselves:
Nerf by Skill Area -> From Full Board to 3x3 Grid
Some Nerf on Aura Skill - Elementalist Kiera used to Buff 35% MAG based on her MAG
Nerf by new DEF mechanics -> Now DEF also Mitigates MAG damage in Arena
Nerf by lowering Skill Damage -> Kiera Elementalist could reach like 200%+ total MAG Based Skill + 2 Turns Burn, they lowered Skill Damage 40% (total) at average
Indirect Nerf by Buffing all Melee heroes by 15% status at average, Mystics, Healers and Rangers didn't get boost.
Now tell me if Mystics didn't get the nerf hammer? You don't see too many mystics on top Arena, maybe you will see some more Ronan now, but he has a mechanic that makes him more useful right now.
About the suggestion, there are some games that do this, mostly when they are more established, those chosen players even tease new content. It's a trend, even more in the era of streamers.
About Real Time Arena, give them time, it is really server stressful to do something like that, they will need a 3x better server for this.