Nexus buff
As the most important objective in the game, I think the reward for destroying the Nexus is too small. For last hitting Dragon and Baron, your team gets a buff. For last hitting towers, you get extra gold. Why isn't there any reward for last hitting Nexus?
I think you should get a team-wide buff that makes you invulnerable from all sources of damage, including the fountain turret. But since that would result in abuse cases, not let anyone to move, attack, or cast spells. Does this sound fair to you, Riot?
