The developer said it plans to add "a couple" of experimental modes every year—they'll arrive as "unpolished prototypes", complete with borrowed art, and players will test them out and provide feedback, which will immediately start to shape changes. The aim is to find some new permanent modes for the MOBA.
Nexus Blitz matches will last around 15 minutes, and each team will consist of a duo jungle and three laners. Its hallmark is that each match will be peppered with map-wide events, described as "mayhem-inducing scenarios that kick off every few minutes". Riot didn't give an example of an event, but cited King of the Hill, Push the Payload, and "the closing circle of death from battle royale games" as inspirations, so it could get pretty hectic.
Winning an event will net your team some goodies, ranging from a massive shield for every team member to a base cannon that launches your allies to the middle of the map. Riot is searching for reward ideas from testers.
Players that go on hot streaks of takedowns without dying will go "on fire", which means they'll do double damage and have shorter cooldowns, but they'll also take more damage, and shutting them down will give the enemy team a big pile of gold.
Alpha testing will kick off with patch 8.16, and will last four weeks. It will be open to all players.
Read Riot's full blog post here.