They've established that Daxamites, Khunds, Dominators and Durlans are all present in the past, so the team could include members of those races (or, from the old Invasion event, people who were modified by Dominator experimentation, such as Josiah Power, of the Power Company).
The Controllers have interactions with 21st century Earth as well.
There's no reason why Bgtzl couldn't exist in the 21st, and a person from that dimension could end up functioning much like Phantom Girl.
Whatever swallowed Jo Nah and infused him with Ultra-Energy could be out there, and accidentally ingest some joyriding alien teenager, creating an Ultra Boy prequel.
Brainiac v1.0 is quite active, and could create AI subroutines to do some work for him while he's busy elsewhere, only to have one of them break free from it's programming and choose to work against it's creator. Brainiac 2.0?
Tasmia Mallor has ancestors with shadow powers, former 'Champions of Talokk VIII.' Perhaps she's not the first one to go off-world?
The Atom could end up 'discovering' Imsk.
A stony figure called the Man-Mountain might be able to grow to great size. Later on, he'll turn evil and in a colossal fight with his former team-mates, his inert body wil be thrown into space. In a 1000 years, he'll end up near Mars...