Honestly, Marvel are saving the best villains for when RDJ leaves. Namely Zeke Stane, Living Laser, Madame Masque and the real Mandarin.
I like the way you think.
Not to mention with Spider-Man in the fold a whole new world of possibilities opens up. Spider-Man has such a terrific rogue's gallery. I would hope Marvel will take great care with how they handle Norman Osborn. He is easily a Top 5 Marvel villain IMO. Based on what they've done with Kingpin I have great faith in them pulling it off.
Not to mention Kraven, Mysterio, Doc Ock, Venom, etc.
But Marvel in general needs to stop killing off all of their villains. That way even if a villain misses the mark or doesn't have enough screentime to shine they can always course correct in a later film. I think it was a mistake to ice off Ronan and Kurse. I think it's a bigger mistake if Ultron is truly gone for good. Fortunately, Ultron is the easiest character to bring back out of all of them.
I firmly believe that Red Skull will return at some point in the future. And they absolutely should. I wouldn't consider him a top 5 Marvel villain but he's definitely in my top 10. And he's evil as $#@! Whether he's played by Weaving is irrelevant. Under all the makeup they can make it work with another actor. I'd hope Weaving would reconsider though. If the Russos are still at the helm I think you can count on a pretty defining version of Johan Shmidt.
But Marvel absolutely needs to plan on the day RDJ hangs up the suit. Thankfully they have GOTG and Cap both as very healthy franchises at the moment. I loved TDW but I'm know I'm in the minority on this site. Thor needs Ragnarok to hit it out of the park and gain back fans. I really hope it does because Thor also has a great rogue's gallery that hasn't even been tapped beyond Loki. Malekith to me was a throwaway villain but I sure loved Kurse but he's dead. But Amora, Skurge, Surtur, Ymir, Mangog, Hela, Karnilla, Ulik, the Wrecking Crew, High Evolutionary, Ego, Gorr, Set, Pluto, Ares, Fafnir, Desak, etc. are all potential quality villains for the future.
Future of other franchises remains in doubt. Ant-Man turned out really good but I'm worried it's not going to find its audience because so many people wrote it off before it even hit theaters. Right now they are saying it may not even make #3 this weekend.
If Ant-Man underperforms that decreases the chances of a standalone Hulk film as well since Marvel would need to share profits with Universal on that. That's a disappointing prospect especially because I really would like to see the Leader in the MCU. He's never even been referenced once since TIH. Meanwhile, where is Abomination??