I think Ultron and Red Skull are both real easy to bring back. Ultron could be hiding in someone's cellphone from a story perspective, and Red Skull's "death" seemed like anything but, visually. Those are definitely 2 villains I could see being revisited in the future and made more powerful.
Say Red Skull was teleported by the Tesseract to a far away warrior planet where time moved slower, and over this long period of time he acquired different powers from various enhancements/artifacts and just interstellar combat experience. Ultron could simply take advantage of the numerous tech advancements that have happened in the MCU since Age of Ultron, or something left behind by Thanos, and reform himself in a similar form as Vision. Only way more menacing.