I have been thinking about this ever since Karasakal mentioned it in the February 2018 PM letter. However, there haven't been any further elaboration on his part as far as I'm aware. I sent in a question about it for the Q&A but it wasn't answered (unless I missed a short mention of it). Right now I think the team is unsure about whether or not this is a good idea, but I hope I can help change that. I hope I can make them say "Yeah, that's that's a really good idea. We should do that."
So why do that?
Because here's the thing: As we all know, the game is Pay 2 Win and that negatively affects both the current playerbase as well as new players. However, removing p2w outright at this point in time isn't really an option. But we can take a step in the right direction. The first step IMO should be a PEN-stat room option for all modes. The benefits this would bring includes:
An easier time setting up fair matches. Getting players to your room can be tricky even without requiring the players to go through extra work. Right now, you'd have to make sure everyone in the room goes to the in-game shop and gears up in PEN stats and then you'd have to have enough players doing that. And your players have to join with trust that others will do the same, because if they don't it's a waste of time. The only variant of this that is viable right now is S5, which brings me to my next point.
Fair matches don't need to be competitive. With it being easier to set up and get players to PEN-stat matches, there is a much bigger chance of casual PEN-stat rooms popping up. Also, you'd be able to use costumes you've bought in these rooms. Besides, most new players would probably be playing casual matches, which brings me to my next point.
New players. S4 needs more players, but having to deal with p2w doesn't make a good case for them to stay. Having at least an option to play without it could increase the amount of people who stay when they try the game out. IIRC it was said on the Q&A that Aeria have been holding back on advertising the game before they got everything working properly (correct me if I'm wrong). This room option is something you'd want to implement before putting in the big guns in advertising.
Weapon balance. With a more consistent environment in terms of damage and such, balancing weapons should be much easier. Besides, simply having PEN stats balances out bombs as you can only have one per life.
We have already got code for it. There already is an option like this for the TD-pass gamemode, so it should be fairly easy to port to other gamemodes as well.
I think a PEN-stat room option would be a key step going forward that could potentially save the game, but right now what we need is a discussion about it. If this is going to get done it needs to be on the radar.