Not every character needs a solo film. Black Widow doesn't need it. She's been fine getting development through Ironman 2, Avengers, Captain America 2, and now Avengers 2. Just keep at that, while introducing new supporting characters too. So now after Ultron, maybe her next big supporting role would be in Black Panther. Gives people on the fence about seeing it a reason while keeping the universe more connected. Introducing side characters that get their development by appearing in multiple other priorities. Not just for a Cameo, but as actual supporting roles.
Hulk too, but for different reasons. There is no reason to do a solo film for him anytime soon. Maybe have him as a bigger role in Captain 3's Civil War or something.
It's a shame Hulk movies are so hamstrung by what the vast majority of people seemingly want from a Hulk film. Him smashing shit up.

I was thinking back to how magic that simple scene in the Avengers was of Bruce Banner and Black Widow in india. Him outputting a mad intensity that he could switch anytime.
A film built around that understanding that the Hulk is a terrifying presence when unleashed would work well. I loved the Ang Lee film because it understood that.