I disagree that Dr. Strange has to be an Iron Man-level successful character for Marvel's supernatural plans to unfold. I mean he has to be successful, of course, but Iron Man-level success is basically lightning in a bottle. I think Marvel would be more than happy with Thor/Cap-level success, which is to say, "very successful".
Here's how I would do it...
Dr. Strange is a given at this point. Assuming it performs up to expectations, we'll be getting a sequel wherein Blade and Ghost Rider can be introduced. Not exactly cameos, but not to the extent of them being major supporting characters either. Basically touch upon their respective backstories either via short montages or exposition. The intent here is to introduce the characters and give each one or two really cool, important scenes, leaving the audience wanting more.
Then, bring on Midnight Sons, which is basically Supernatural Avengers. The focus, of course, will be on Dr. Strange, but Blade and Ghost Rider will have huge roles as well. Along with some others to round off the team. After this, depending on audience reaction, solo movies for Blade, Ghost Rider, etc. can be planned.
All of the above is one option. Another option would be to squeeze in Blade and Ghost Rider solo movies before the Midnight Sons team-up, but that would depend on Marvel's level of confidence in giving them major tent-pole solo releases.
If I were Feige, I'd go with the first option, but that's not to say that the second option isn't viable.