Since quite a few of abuse cases come from conc potion (stacking), I think changing it so it would reset your class ability cooldown after 5 seconds would be a nice rework. No weird stacking, and a lot more value for longer cooldown class abilites like IB taunt or Merc Stagger.
After all the potion would be a lot more desireable for many classes.
Cool, but it will be useless for a lot more careers, which have weaker actives.
It speeds up recharge, instead of resetting an active for a reason - because they are balanced by their CD. This way you can use several weaker actives when drinking this potion, or one strong one - and that’s perfectly fine.
Also imagine using such a pot when your ult is nearly recharged.
Just make your ability unusable while the last one is still active. Still no weird stacking, and no need to change Concentration pot 
Potions are fine as they are. Leave the items alone, balance the classes themselves. Tweaking items so certain carreers cannot “cheese” them anymore always results in hitting others even harder, which in turn makes the ones you want to nerf stronger in comparison.
Why exaclty do you need 5 ults in a row? What are the low-cd “weaker” ults: Sienna Fireball, Fire Walk, and dashes. Do you really need permacharging kruber for bosses? Its just so abusive, as is spamming staggering fireballs. Id rather have nice quality over abusive quantity.
Agree with the “not usable while old is active”.
Agree with the class balance, a bit of finetuning is still needed. Still wondering who gets hit “harder” by not being able to spam charge 5 times 
It’s not perma-charge kruber that’s the problem. It’s the glitch to make his handgun kill a boss in a few seconds. I’ll agree with the above post, nerfing concentration potion would hurt a lot of classes that don’t benefit from ult stacking glitch.