I'd rather keep the solo films separate from one another. There's no need to throw in little winks and nods to Superman's existence in a Batman movie. When I see a Batman movie, I'm there to see a Batman story. When I see a Flash movie (God willing), I'm there for a Flash story. I've enjoyed the Marvel universe for the most part, but I admit that the SHIELD/Avengers subplots sometimes got in the way of the main characters' solo efforts.
But by the same token, there's nothing to suggest that these characters don't co-exist, so you could have these individual franchises and still have them come together in a Justice League movie. The connection just doesn't have to be as "in-your-face" as Marvel's strategy.