Dick Grayson had to be in his mid-20's, but he didn't have to be Nightwing. They could easily have had him show up as Robin, with a redesigned suit. Maybe a suit based on the Earth Two Robin suit that was in the comics in the 70's. It would have been more appropriate to the era than Nightwing.
I understand what you mean about the Joker thing not keeping with the tone of the show. Somehow that didn't bother me as much. The Joker had been a fairly dark and violent character when he was introduced in 1940, and in the 70's they were going back to that interpretation and building upon it. So I could kinda imagine that, maybe a decade later, Cesar Romero's Joker would go off the deep end and become a killer. I mean, its a story they could hypothetically have told had the show been revived in 1977! Not so with the Nightwing thing...
I haven't read much of the '66 Batman comic beyond that crossover so I dunno about the Bane and Harley Quinn thing. Must check it out!
But yeah, getting back to topic - I really don't think I'd want to see a '70's Doomsday' in this particular series...