I think part of the inherent difficulty with implementing a "middle ground" version after Story and Savage is the rewards.
Most people "try" Savage because it offers the best gear in the game. We've seen this before, once Tomestone upgrade tokens are available from the 24 man, the number of Savage runs will sharply drop, or at least Savage progression.
In general, if the hardest difficulty does not also provide the best gear, then most people wont bother with it at all, aside from the "Super Elites" who will clear it once during the World First races and be done with it.
The question becomes, what can we offer to the hardcore players to make them want to repeatedly clear the most difficult version?
- If we make the ilvl of Savage 10 higher than that of Normal (Normal being tougher than Story), then a lot of people will opt to skip Normal if they can kinda clear the earlier parts of Savage, or will farm Normal and ignore Savage entirely due to excessive difficulty.
- Suppose we make the ilvl the same? Then we need to somehow reward the Savage clears, say Savage drop 3x as many chests? Or each clear of Savage gives you 4 tokens instead of 1, so every clear you can be rewarded for your efforts. Or simply tack on the "Augmented Glowing" effect to Savage drops, so when someone who's rocking Savage gear walks into Idyllshire, all da gobbies be like: "Damn, we got a badass over here!"
Personally I would lean more towards offering a greater volume of rewards for clearing Savage, and apply a similar methodology to tuning Normal based on Savage.
For example, suppose we design Savage with a set of mechanics, which have a frequency, damage, and execution duration (how long you have to deal with the mechanic before you fail).
For Normal, cut the health of bosses down to 75%, mechanics frequency to 66%, damage to 75%, and increase execution duration by 50%.
So if a Savage boss has 4M hp, its Normal equivalent would have 3M. A tank buster that comes every 100s would come every 150s. Said tank buster in Savage would have a base damage of 30k, in Normal it'd be 22k. And if in Savage you get targeted for a mechanic and have 10 seconds to execute it, in Normal you'd have 15.
Same general fight, just easier. And in being a toned down version of the Savage fight, it will better prepare people to deal with the "real" mechanics in Savage.