Avenger 3 being split into 2 films is something I've speculated on since almost 2 months before that rumor started.
The Thanos Imperitive: Avenger 3 pt1
The Avengers 3 pt2: Infinity Guantlet.
Part 1 is where Thanos shows his might, collects the stones, probably starting with the familiar Asgard and working his way down to Earth, which not only causes havoc all over the galaxy, but leads right into Part 2 where the Guardians and the Nova Corps and everyone else seeking revenge and to stop Thanos end up following him to Earth. (somewhere in here, Squirrel Girl is credited with distracting Thanos just long enough for someone to deliver the winning blow to Thanos).
As far as Spider-man goes.... I wouldn't mind Garfield, but can we really fold ASM1 and especially 2 into the MCU as it is? Can they just be allowed to use the Spider-man character w/o actually cross promoting? Not using ASM, but maybe a new MCU Peter Parker? Sony is probably in desperate enough shape to agree to that. ASM2 was a hot mess, I have almost no faith in a Sinister 6, Venom or ASM3 movie from Sony at this point, unless Marvel lets them keep the property, but Marvel gets a hand in crafting it from here on out and a percentage of gross or something.