Before anything more Sonic 4 comes out, I’d like to share a personal headcanon that the Robotnik will be seeing in movie 4 won’t be the main timeline Robotnik who ended up sacrificing his life to save Stone
This take on Robotnik would be a "what if" sort of deal
This alternate timeline of Robotnik predates all the way back since the events of the first film, Ivo being someone who heavily relies on machinery and himself over anyone. Everything goes as what happens on the first film, G.U.N. notices the existence of Sonic due to the power outage he made at the baseball field, They hire Robotnik and Stone as contractors, but here’s the thing that occurs, Robotnik ends up capturing Sonic and knowing how much power he can do thanks to Sonic. He ends ups keeping Sonic for himself and starts making off from him until eventually he turns him into a living battery, thus Metal is born.
Ivo ends up turning against the government with all his new powered machines and knowing how much capable he is against anyone that opposes him, thanks to Metal. As the years go on and mind you, this Ivo doesn’t fully understands humanity bonding compared to main Robotnik and after him capturing Sonic, him reaching his world domination goal, he starts eating uncontrollably, not because of depression unlike what happened on Sonic Movie 3, but for this reason is for pride and ego, he shaves his head off, he lets his mustache grow immensely, a bit more than main timeline Rob. He starts sporting his Genesis/Classic outfit and let’s the “composed” self away, so he can feel himself stand out from the crowd and let everyone know the person who once was a human being hurt by society, to end up their new emperor of their world. He sports the nickname Sonic gave him at San Francisco, once being a mockery to ended up being a nickname, a symbol of fear for their new leader. This alternate version of Robotnik would be referred as "Eggman" at that point moving forward. (Hell, we might even get to see the same interpretation outfit that was shown briefly in Sonic Movie 2)
He will pretty much have the same type of personality, mannerisms from the first film, but amplified by a 100% or even more, still heavily relying himself with himself and his machines.
This could work when it comes to bringing Ivo into full blown villainy without erasing the actions of the previous films of the main timeline Robotnik, specially his sacrifice and his arc with Stone.
And due to how power-hungry he is, not content about only ruling planet Earth, he starts seeking other planets to conquer over, one of those being Sonic’s home planet, being briefly teased in the first film, the teaser and also the Pre-Quill Comic showing other locations from the games. Eggman begins capturing the animal critters that they get to live in that planet and starts using them into Badniks to match even more the quirky designs of the games. And builds his based of operation in that planet, which either he could go for Eggmanland/Scrap Brain/Metallic Madness.
(Here's some of the zones that had been shown in the comics and the existence of other animal critters like the main cast)
Just like Mania where he has a machine to mass-produce Silver Sonics he does use it to make copies of the original Metal Sonic and one of the survivors and still standing against Eggman was Amy Rose, with the hopes of finding someone who can aid her she ends up where Movie 3 left it off at the post credits scenes, asking for help from Sonic and Co. to fight against Eggman and stop his reign of terror
Thinking a bit more closely, in some way this could also work if they ever wanna tackle the idea of a Generations type of plot of having two takes on the same Robotnik, just different timelines but different complexities of the character.
Dr. Ivo Robotnik = (Modern Outfit Movie 1, Movie 2 Movie 3)
A man who craves for recognition
Has a personal grudge against Sonic
A misunderstood genius in a world of negligent humans that only uses him for his intellect or see him as a threat to society
Sleek designs for Badniks
Lacks the use of the nickname “Eggman” (only exception on the VR bonding scene) and maintains his full real name
Becomes overweight due to depression after the losses against Sonic
Dr. Eggman = (Classic Outfit Alt TL Movie 1 - Movie 4)
A man who thanks to his victory against Sonic his ego takes a huge leap and becomes an integral part of him
He’s fully detached from everyone and heavily relies on machines instead of humans (same mindset from the first film)
Power-hungry dictador that wants to colonize more locations than what he already has
He has everything he ever need
Only human in a world full of animal critters
Cartoonish designs for Badniks
Lacks of use of goggles and uses the Pince Nez glasses
Becomes fat to show his immaculate pride
He embraces the nickname of Eggman to cause tremor