USS Defiant (NX-74205) will have a time travel accident, rendering the whole crew unconscious, and Discovery crew will beam aboard to investigate.
S2 speculations: Spock will appear. A young Solok will be the new captain (kidding, but boy would that be funny!)
Probably everybody else has said that, but: Spock will conveniently be on a temporary assignment when the Discovery hails the Enterprise just so that he wouldn't be able to meet his dad. I'd speculate he'd be helping out with analyzing data from the spore drive but he'd be as far away from the Enterprise as possible.
Unless they can get Quinto to cameo, in which case Spock is there. Wonder who they’ll get to play Pike?