All it would take for Arrow/Flash to be in the same universe as Constantine is just a little nod some thing along the lines of a mention of a cult that has healing powers in the south or something similar. They don't actually have to crossover like Arrow and Flash when was the last time Constantine teamed up with ether of them in the comics?
For Supergirl Iris could go investigate the new "Alien" and have a bit of a crossover there.
But for Supergirl it would make sense for the Crossover to be more direct as everyone would be extremely concerned with an invulnerable Super powered Alien flying around as opposed to a con man who may or may not have magic powers.
The leads of the show only need to get together for something truly special we don't need it every year otherwise it will start to lose it's allure.
@frozen: The Atom will have his own show. Vixen will join Arrow to replace Felicity as the main female led as she goes to The Atom.
Flash will continue to be amazing 2016 will see the first 4 part cross over featuring Arrow , Flash , The Atom and Supergirl. CW will pick up NBCs Constantine and create a time slot call DCW cause marketing.