If the Flash is introduced on Arrow
I hope his own series is fairly soon after that. Once Oliver Queen became a regular on Smallville
there was no chance of a Green Arrow spin-off. He had been so exposed on there and tied into its mythology he would not have worked in a show tied to it. Having a connection and using Arrow as a launch is good. But I think it would hurt both shows if they are too intertwined.
For this to work both series will have to have separate styles. Also I hope Central City will not be next door to Sterling City. That way even if occasionally superpowered heroes and villains appear on Arrow
it can still maintain its own unique style and identity.