Ive played brand mid a lot and personally? I think the only buff he needs to become relevant again is a passive change.
Id say keep the explosion, make it 2 seconds to detonate instead of 3, BUT have no scaling on it, and make it a flat dmg number. Like, say 8% of max hp, not as strong as what it is currently, but give the scaling back to his DoT. That way, you can keep the aoe explosion, but have brand be what he used to be like and actually have a presence in mid lane without having to use all of his shit to do maybe a half hp bar of dmg.
He would be able to rely on his DoT again rather than try his best to shoot all his spells out before he gets obliterated in a fight, especially since you can miss 2 of them or have really bad luck with ult bounces. Fights where this happens can make or break a brand, depending on if hes allowed to play the game or not.
I also say this because with the amount of mobility and dashes in the game, its actually surprisingly difficult to land a q if it isn't point blank, especially if you are going for a stun since the enemy knows that you are trying to go for it. With this buff hed actually have a chance to do dmg in lane but also still be relevant lategame. Thoughts?