system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] How would a Mechanicus Imperium of Man look like?

So basically: Entire 40k verse is the same, but the Emperor gave Mars full control of the Imperium after the HH.
gpt
With your edit:

The Imperium of Man would be kind of screwed up.

One thing that isnt really talked about much is how the Imperium of Man is like a Feudal state. You have the state, then everyone under you (the planets and the like) give tithe in exchange for the 'legitimacy' of their rule (the planetary governors). This ensures that something as large as the Imperium with a million worlds can actually maintain the current status that it has. It's both the reason why things are so stagnant and also the thing that ensures the Imperium actually can survive.

The Mechanicus on the other hand is more like a coalition of sorts. While Mars is famous... they arent the central authority. Mars maintains it's dominance by 'leasing' out STC printouts from its massive vaults, a larger number of them than any other Forge World. 

In fact each of the Forge Worlds are their own state under the banner of the Mechanicus and treat each other in the same way that an independent state will. That is with a long arm, any kind of sharing, any kind of cooperation, anything at all is legalized, contractualized and every 'ized' that there is. Anything else quite simply impossible for them since each guards their secrets very well indeed. 

Remember that many Forge Worlds have existed since before the Great Crusade and were in fact found over time as the Crusade reached them, each one treated like a valued ally instead of a vassal. There are no demands, everything is done through treaties and contractual promises. 

For example some have STCs for certain designs like plasma, some with Meltas and some with lasers. This secret keeping has consequences where Forge World will actually go to war with each other over technology, with Mars actively raiding other Forge Worlds to steal their STCs and maintain its superiority, sabotaging all the while. 

Therefore an Imperium that is ruled by the Mechanicus will not be the fractured but whole Imperium that currently exists, but rather one that is formed by countless small states that are always in constant competition and combat with one another trying to find the way to the Omnissiah at the cost of all others around them.

The Tyranids appearing out of nowhere, the Necrontyr, the Eldar, the Dark Eldar, all of it wouldnt be met with a centralized and massive reaction, but rather politicking and infighting over rights of salvage, over who sends what and how. All the while the Imperium itself is being nibbled away constantly while the Forge Worlds have very little desire at all to expand. The chipping itself becoming permanent over time. 

The Imperium in other words might have the big guns, but they will be very ineffective and locked away for a rainy day guns. Given enough time the Imperium will either be torn apart in a massive civil war... or just destroyed from the many many enemies from the outside. 

A Mechanicus Imperium therefore is a very bad idea for any kind of long term stability and will doom it to destruction due to the lack of any kind of central authority to force reactions for the greater good of the whole at the cost of losses in some segments. 

Sorry for the poor quality of the reply, if you want clarification feel free to ask for it.