i am fairly confident that there are already talks behind the scenes in the various competitive leagues about creating a promod. whether it happens or not is up in the air
from my understanding a lot of RGL's projects specifically are already decently ambitious, and im sure that there are motivated people willing to put in the work (and id like to think a project of this size would have money behind it)
using the promod as a balancing tool should be the last priority of whoever would take up the mantle of this project. QoL changes, potential anticheat improvements, baking dm server queues and maybe even pug services into the client, all seem very interesting and i think these are all more pressing matters than balancing
i also think using the promod to improve the viewing experience of competitive tf2 would be awesome. obviously there should be improvements to production tools, but beyond that, im personally imagining some kind of way to spectate matches live in STV, similar to how you could spectate CSGO major matches while having the caster's voice broadcasted through the STV.
also, in the same vein of CSGO, potentially having the demoui functions of CSGO/2 would be literally the greatest thing ever, and this feature should be like 50% of what would make a promod appealing to established competitive players
as for balancing changes (despite my belief that it should be the lowest priority), i used to believe that sniper and maybe even engi should be outright banned to improve the pace of tf2. but now, with the possibility of a promod, the idea of applying damage falloff to the sniper rifle and sentry and/or applying statistical nerfs to either of classes intrigues me a lot