Testable and Implementable Comeback Mechanic
New Effect: Killing Baron also grants team gold equal to 60% of the gold deficiency of your team. So if the enemy team has 1000 more gold, your team gets 600g total. If your team has more or equal gold, no gold is given.
New Effect: Killing any dragon also grants team gold equal to 30% of the gold deficiency of your team. So if the enemy team has 1000 more gold, your team gets 600g total. Elder dragon could be pumped up to equal baron as well.
I don't know if the numbers are right but this should cause winning team to still think about objectives seriously instead of playing safe and keeping their lead and allows losing teams even more comeback potential.