X-Men: Days of Future Past was a party that everyone was invited to, no matter which time (or timeline) they came from. In theory the upcoming sequel X-Men: Apocalypse should be a little less densely populated since it is only set in the 1980s and doesn't feature future versions of any characters. However, between the introduction of young versions of Storm, Cyclops and Jean Grey, and the addition of newcomers like Psylocke and Jubilee, it looks like the party is still rocking.
The return of Scott Summers raises the question of his exact relationship with Alex Summers A.K.A. Havok (Lucas Till), who was introduced in X-Men: First Class. In the comic books Alex is Scott's younger brother, but since Alex was already a young man in 1962 he is much clearly older than Scott in the movie universe - though not old enough to rule out the possibility of the two being siblings.
It now looks like we may get answers defining the relationship between the Summers boys in X-Men: Apocalypse, as director Bryan Singer has confirmed on Instagram that Till is signed on to reprise his role. Havok was among the first batch of recruits to Division X - and aside from Beast, is the only recruit to have survived and (presumably) maintained his allegiance to Professor X.
Havok had a minor role in X-Men: Days of Future Past alongside other mutant military recruits like Toad and Ink, who were about to be kidnapped for testing at Trask Industries when they were rescued by Mystique. Havok was last seen getting shipped back to America, and will be around forty years old by the time of the events in X-Men: Apocalypse.
Much like it did in X-Men: First Class, Charles Xavier's team in X-Men: Apocalypse will skew very young, as Cyclops (Tye Sheridan), Storm (Alexandra Shipp), Jubilee (Lana Condor), Jean Grey (Sophie Turner) and Nightcrawler (Kodi Smit-McPhee) are all teenagers. The most likely explanation for this is that they are all students at Xavier's School for Gifted Youngsters, who get drawn into the fight against ancient mutant Apocalypse and his Four Horsemen.
Perhaps Havok will be working as a teacher at the school when we catch up to him in 1983, since he's an old friend of Charles' and has plenty of experience learning to control his abilities. That could certainly make things awkward for Scott; sibling rivalry is completely unfair when your brother has the power to give you detention and extra homework.
Singer also recently posted a photo of himself scouting locations for X-Men: Apocalypse, which means that filming will most likely begin within the next few weeks. Can we squeeze in any more casting announcements before then?
Fantastic Four opens August 7th, 2015, Deadpool on February 12th, 2016, X-Men: Apocalypse on May 27th, 2016, Gambit on October 7th, 2016, Wolverine 3 (not the official title) on March 3rd, 2017, Fantastic Four 2 on June 9th, 2017, and some as-yet unspecified X-Men film on July 13th, 2018.
Source: Bryan Singer