They are NOT including another "Invasion" event or similar ones.

Riot has already stated in a /dev post that they will NOT be making more of these awesome gamemodes because more people play URF and TFT than they ever played Invasion/Odyssey. While I dont personally believe them, im just reiterating what was said so the same posts stop being made. Trust me I really want those types of gamemodes back because thats the most fun i've ever had in League. But those modes were hardly monetized like TFT was, so as always they will follow the money unfortunately like companies do in general. And the reason Riot likes URF so much is because its sheer simplicity brings in flocks of players that dont even play the game otherwise, while also catering to any rank(even Iron) because of how braindead the mode is which in turn means more people playing it.

Lets not forget probably the biggest part... that these gamemodes take the most amount of work to make/balance, which is why they also are deleting Twisted Treeline, because they'd rather say it's impossible to balance and has a low amount of players instead of paying attention to it(which would in turn bring up Twisted Treeline's playerbase).