Gamemode Concept: Dragon Onslaught
Now that the technology is already here with the new Herald, I think a Gamemode located on the Rift, but focused even more around special objective taking would be interesting.
In this Rift, Dragons will spawn every 5 minutes or so in random places, but they will be more than the ones we already know, and will have more tricks up their sleeve. A roar will mark the spawning, and they will be shown in the map for everyone that wants to go hunting, something not only risky cause of the dragon itself, but for the possible ambushes.
There will be normal dragons, then of mixed elements, similar to elementalist Lux's elements, then dragons related to nations (Void Dragon, Desert Dragon, Celestial Dragon, Petricite Dragon and so on), and even related to skin lines (Dark Star, Mecha, Star Guardian, Blood Moon), each with special movesets related to their looks, to make them harder to kill as a boss, similar to Baron,.
Slaying them will drop a medal anyone can take, with can later on be kept to get some bonus, similar to the Herald's, or used to summon the dragon to push a lane for you or even to improve a structure, and while this seems stupid when you could push with some dragons, the thing is that killed dragons or turrets will drop the original medal for anyone to take, so in some situations it will be wiser not to waste your hard earned medals in lost offensives.
Lategame, with all the slots full, taking a medal will be somewhat hard, so teams will probably use them to summon a dragon with skills, or another to defend the last turret (or even the nexus) with the dragon skills that it grants to that structure.