The lightbringer change is very bad imo. I'm sure Xypherous has spoken at length why a mandatory must buy champion specific hard counter item is always bad.
Honestly you just need to make an exception to map specific character balance. Teemo's only problem is the sheer density of his shrooms, increasing the cooldown and/or decreasing the duration of shrooms is all that's needed. Do that and he keeps his ultimate without it being such a problem, it'd just drop down into the 'annoying' category that it is in SR.
With the new lightbringer you're essentially removing Teemo's ultimate from the game which is NOT the solution. Not do mention lightbringer is just an all round bad item so you're telling ADC's they have to gimp their build in order to remove a champ from the match while AP, tank etc are still left with nothing. Think about it, you're buying an item with armor to counter an ultimate that does magic damage, nothing about it makes any sense.