WWE needs to just stack the match with as much top talent as possible and as many realistic permutations with the outcome. They did a very good job this year. Taker, Braun, Leanar, Goldberg, and then the surprise of Reigns made this year's Rumble hard to predict. We mostly forgot about Randy and he ended up winning.
Similar to what they did with Reigns this year, ideally I would like to see them get more of the big names from the earlier bouts in the match. It would have been a huge thrill to have AJ come out this year or DB to come out a few years ago. Neither needed to win, just sell themselves as a tough guys who want the Mania main event that badly. Maybe Vince only sees Roman as that guy.
Beyond that it is hard to come up with a stipulation that would work without convoluting a simple concept with years of tradition. I agree with the OP that it feels like something could be done. An Extreme Royal Rumble might be fun. Raising the ropes? Adding a five second count out after the two feet hit the floor would be a change that could lead to some entertaining spots. Maybe some kind of benefit to any competitor that lasts a certain number of spots. Like if number 1 lasts until number 8 they get a Singapore cane. In the other end of the spectrum, removing the Mania main event stip makes it easier to believe someone like Mojo Rawley could win.
I don't know, these all kind of suck. Any idea just seems to convolute something that is simple and works for the most part if you are emotionally invested in enough competitors and the idea that the Mania main event is as important as WWE tries to make it.
My advice to all tenured fans is to govern your expectations. It is still just a battle royal so there can only be so much action. Realize that there is only so much WWE can do with it while still making the money that they want to make. Remember that it is long and can drag out for much of the match. There is no guarantee that you will get a surprise that you want. And if Kofi does not win, we riot.