Here's some possible scenarios post Thanos:
-Expand on the Eternals, throw in Nova, Quasar, Adam Warlock
-I *think* Ghost Rider's rights have reverted, so either Mephisto or, with Dr. Strange, Dormammu in the demon realms
-Cassie Lang is rumored to appear in Ant-Man: set-up Young Avengers, especially if the likes of RDJ will be retiring soon.
-Asgard witnesses Ragnorok, ensuing chaos somehow creates The Wrecking Crew (as per their origins)
-The Worthy(?) show up - Black Widow, Ms. Marvel, She-Hulk, Maria Hill, whoever, is possessed by a hammer and becomes "Skirn, Breaker of Men" (among others though this is the only one I know about for sure)
-World War Hulk and Thundra, Beyonder and the Secret Wars (together?!)
-Inhumans (seemingly greenlit last I heard), Terrigen Mists, Kamala "Ms Marvel" Khan, everyone gets super powers, chaos ensues, possible new origins - and copyright loophole - for "X-Men who are also Avengers," maybe even Spiderman if his rights *somehow* revert during that period, if not Peter Parker than at least Miles Morales. Because Scarlet Witch exists, can potentially roll into modified "House of M"
-When the cast gets big enough - Civil War and/or Secret Invasion (assuming Skrulls aren't limited to FF4), works in GotG (which I've never actually understood how that could work: between rendering Groot and Rocket and paying RDJ's salary, the budget left over won't even cover a buffet lunch for the film crew).
-AIM, HYDRA, Maggia (assuming they appear in Netflix Defenders alongside Kingpin) have worldwide gang war, She-Hulk acts as defacto liaison between street justice Defenders (knows Matt Murdock personally through law) and Avengers through Banner, possible rollovers with different regions and heroes - Black Panther, Captain Britain and Betsy "Psylocke" Braddock and Excalibur (no Nightcrawler, sorry), West Coast Avengers, discovery of The Pride and The Runaways.
-Reboot the franchise as new continuity, unexpectedly crossover with current one, mash them together into new fused storyline
-Time travel creates new origins, creates new team dynamics (what if Cap goes back to WW2, finds Peggy, they have secret child/grandchild who becomes US Agent or a new "Bucky" or takes up mantle of Winter Soldier after original inevitably switches sides), brings back old characters: opening for Red Skull?
Oh I know I'm forgetting something - there's no way I'm not.