IF the imperium dispand i think it will be because of chaos, corruption in the space marines. Chaos will control the galaxy, chaos will control imperial worlds and the other armies will either side with the chaos or be exterminated.
it will come down between orks, necrons and nids.
The orks will begin to take over most imperial held worlds, the nids will start eating them and the necrons will start harvesting the souls of humanity and move to the orks.
Chances are, nids won't fight necrons because they have no organic matter on their worlds anyways. but the necrons will initiate that fight between them
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