There seem to be two divergent approaches to this.
1. Replace remaining lame attractions like Ellen, Imagination, and Innovations with ... something good: thrill rides, ideally based on movie franchises, or shops or characters. Things that are popular at other Disney parks, but with no focused vision for what Epcot is about.
2. Bring those lame attractions back to their former glory. Bring back Dreamfinder. Bulldoze the vomit comet and rebuild Horizons.
Neither of these things will really work. #1 will result in, at best, a clone of Tomorrowland, a land based on science fiction instead of science. At worst it will leave a bunch of rides without a relevant theme, when the movies they're based on don't age well. It may be a park that teenagers will go to to ride thrill after thrill, but it won't appeal to adults and young kids. It won't be a family park as Walt intended.
#2 will please nostalgics, and I'm one, but I'm not sure people seeing it for the first time today would connect with the original Horizons. Epcot needs to get back to the idea of inspiring people about the future with science and technology that is possible but not yet realized or not yet mainstream. Inspiring is the key word, not thrilling or entertaining. An attraction should have those too, but without inspiration, an attraction at Epcot is hollow. But we should not just go back to the particular vision of the future that we had in the 1980s. We need to figure out a positive vision that makes sense now, and plan for changes as that vision becomes outdated.
I think Future World probably needs a comprehensive plan to replace everything that's there now, not all at once, but one at a time working toward a cohesive theme. We shouldn't limit ourselves to improving individual pavilions within their existing themes. If a theme is no longer the most relevant, change it.
If it will get people behind the idea, I don't mind retheming it as Stark Expo, as long as the technology message remains realistic and not fantastical. I don't want to see anything powered by tesseract.
Since I think there should be a major rethink, I'm just going to say what is not working about the pavilions, not how they should be fixed.
Spaceship Earth - the one pavilion that doesn't need and shouldn't have major changes. It's a history lesson, and history doesn't change, it just gets added to. The descent portion can be planned for frequent updates, covering communication innovations since the ride was originally built. For now, things like the Internet, mobile phones, and Twitter. In the future, who knows?
Energy - this was always a little weak on theme. What do dinosaurs have to do with energy? Okay, I guess I can see it, but it's a stretch. It was originally a very inspiring show, though, even if it mostly inspired you to be a geologist, not to invent alternative energy. The addition of Ellen makes it too comical to be inspiring. Just do something completely different with this space.
Mission: Space - Mars just isn't quite mysterious enough. This could be rethemed with a new mission, based on real science but with more stretching of the imagination, like a trip to a planet around another star.
Test Track - The problem is, how stuff's made is just not that exciting a subject. I'm not sure a car-oriented attraction really has a place in today's vision of the future, unless it focuses on alternative fuel vehicles. Or it can take on the broader subject of transportation in general, and how we can build a system of transportation and lifestyles that have less impact while still making it "fun to be free".
Innoventions - Right now this is like a corporate promotion fair. This needs to get back to something more like it was, with exhibits sponsored by real innovative companies. (What is an insurance company doing here? IBM is pretty innovative, but show off some of your AI technology that's winning at Chess and Jeopardy, not some lame trivia display.)
Seas - pretty good as is. You can't go too wrong with a giant aquarium. Needs minor updates of exhibits over time, that's all.
The Land - Soarin' kind of distracts from what was the original message: how the land can provide for our needs without us impacting it negatively. I could see how Soarin' could connect to this message, by showing us why the land is worth protecting. It needs a pre-show or post-show to drive home that message. LwtL seems outdated. I don't think hydroponics is really the hot topic in agriculture these days. I think this just needs to be replaced with a new flagship attraction.
Imagination - This one can't be drastically overhauled. Imagination is the "heart" of innovation. Besides, Figment is too beloved to go. Build a new attraction with Figment, and fix the writing for the character so his mischief is the result of his curiosity, not brattiness.