Brimstone is so bad but I have some ideas that could make him substantially less bad.

1. Let him smoke anywhere on the map. I think one reason he's so much worse than other controllers like Omen and Astra is because he can't place his smokes across the map, but rather is limited to the radius on his tactical map. Allowing him to be able to place his smokes anywhere on the map the same way other smokes can would make him a much more viable agent. That being said, I think price should increase from 100 to 150 credits per smoke to balance it out slightly. Same concept applies to his ultimate in that I think it shouldn't be confined to such a small radius, increasing his retake and clutch potential.
2. Stim Beacon rework. One way to make this have a considerably higher impact on the round would be to change its effect from increasing your teams firing speed to decreasing the enemies firing speed. So instead of buffing your team, the stim beacon would nerf the enemies (if that makes sense, idk). This would make Brimstone better at controlling sites similar to Astra's suck ability, Vipers molly and Omens flash which all hinder the enemies. Another possibility would be to make the Stim Beacon's boost go to damage output instead of firing speed like the Damage Amp ability from Spike Rush which also makes the stim beacon much more impactful.
3. Give Brimstone the same floating ability as Jett. Im jk but the list looks better with 3 things instead of 2.

Again this is just my thoughts sorry for cringe, lmk what you all think