This gamemode wouldn't be for the weak of heart. I've played LoL matches that have lasted hours, most last roughly 30 min. This would not be in your typical gamemode and I would rather like it to be a Clan vs Clan or specialty mode. Reason being is that casuals will most likely not stick a match through.
Basically how this would work is both teams have 3 bombsites they must defend. The first team to plant a bomb and destroy all their opponents bombsites wins. Between each bombsite and out in the map sits Sentry guns, IMSs, and other defensive obstacles that each team must fight through.
Each kill, chained kills, streaks of kills, and objective completions earns a player points that they can spend on upgrades during the game. When you start a match you're given options of various classes and each class has a very basic but specific loadout. You can upgrade weapons, attachments, and call in air support and ground support to assist you throughout the match.
The maps for this gamemode will need to flow very similarly to the 3-lane model that we're accustom too. This will allow for fast and balanced matches.
You can YouTube LoL tutorials and get an in-depth look at how the game plays out. I think you'll see that there could be an easy transition between the two games. There's a lot of concepts that are transparent with Call of Duty.
As always, any changes or feedback are welcome. Don't vote with making a comment of some kind.