The way I see it is as follows.
In 1988, John Frusciante replaced Hillel Slovak in the Red Hot Chili Peppers who had lost his life after a heroin overdose. Slovak was not a bad musician but up to that point the RHCP were relatively unknown and had been since 83. Frusciante was a great fit in the band and they developed a style of music that worked well for them which helped them achieve mainstream success with “Mothers Milk” (Halo CE/2).
In 1991 they released “Blood Sugar -Yoink!- Magic” which is now regarded by many critics as one of the best albums of all time (Halo 3). Frusciante who had developed a signature style of play similar to Hendrix was now a key part in the band’s formula.
Frusciante then left to deal with drug addiction at which point they brought in Dave Navaro, guitarist of Janes Addiction. They released “One Hot Minute” which was not on par with previous albums because the forumla had changed and people don’t like big change (Halo 4).
Was it a bad idea that they brought Frusciante back? Is that moving backwards? Should they have kept with Navaro and tried to work with him? No. With Frusciante they have released 4 of the biggest albums of the 90s and 00s (3 of which included in the Rolling Stones best 500 albums of all time) with songs that defined the 90s and 2000s e.g. Under the Bridge, Give It Away, Californiacation, Soul To Squeze, Otherside, Around the World, By The Way, Can’t Stop, The Zephyr Song, Dani California and Snow, to name a few. (This could be Halo 5)
But not one album sounds the exact same. They stuck to the formula however which is why they have been so successful. Halo 4 tried to be something its not which is why it failed in comparison to previous titles.
The little changes like graphical improvement, expansion on customization and sandbox ect is what I would like to see. Maybe even loadouts could work if restricted. Things like Sprint however do not fit into the formula so isn’t worth any effort to try improve upon. They will never be accepted and will always hold the game back.