4. Go Deeper Into The Lore
One strong part of Army of Darkness and Evil Dead 2 was that they delved into the demonic lore just enough to intrigue us without detracting from the story. They gave us enough information to intrigue us with the history of the Deadites and the book, but they also hinted at a lot. For example, where do the souls of the possessed go? What is the demon world like? Did anything happen to the demon inside of Ash? There are a lot of elements that can be potentially explored. Perhaps Ash and company will take the fight to the Deadites this time around. Maybe, rather than traveling to the 12th Century, they'll travel back to the origins of the book. Maybe they'd even, by accident, CREATE the book. Who knows? However, they need to descend into the mystery even further.
3. Practical Effects
Evil Dead is not a franchise that needs a hundred-million dollar budget. Part of Army of Darkness's appeal is its use of stop-motion and obviously fake set pieces. There's even a scene in the flashback at the start of Army of Darkness where you see Bruce Campbell being pulled along by strings! In most films, this would detract from the quality, but here that's part of the appeal. It's intentionally silly. The films want you to know you're watching a movie, and you should just enjoy the ride. It may be tempting to just use CGI for everything, or to go crazy with awesome effects. However, it wouldn't work for Evil Dead. It just wouldn't jive right with the previous entries in the series. If you don't believe me, look at the Indiana Jones franchise. Three films of practical effects, and the fourth one uses CGI for everything. One of these films seems a bit out of place with the rest, doesn't it?