For this lifelong JSA super-fan, the speculation about who will appear is super exciting to me. The JSA angle makes Stargirl the DCU series I'm looking forward to the most.
Re: Mr. Terrific... The modern version is on Arrow but I'd be darn happy to see the original/golden age Mr. Terrific. I've loved him since he appeared (and died) in that JLA/JSA crossover.
Also I don't much think Johns and the other producers of DCU shows are going to let characters appearing elsewhere to keep them from using them. I would have felt much differently if we hadn't just seen a preview involving Batman. He's been off-limits to everyone but the movies. That they're actually using him in Titans tells me all bets are off in terms of available characters.
After the JSA being gone for SO long in the comics (curse you, Doomsday Clock delays!) it's thrilling to me to see them casting an Hourman, Wildcat, and maybe especially Sylvester Pemberton (Star-Spangled Kid). I've missed him since DC killed him off decades ago.