if you know Xiaomi CEO Lei, and his history, you would have an answer is no, they won't develop a new OS instead of Android.
When Xiaomi sets himself on internet company, it lost the far future develop ability, and in fact, Xiaomi sells TV, routes(Xiaomi box) and other products, but progress is stoped by Xiaomi 4, even worse than lots of China local brand smartphones.
Anyway, Xiaomi made his own legend, how to be successful by reducing profit of smartphone, it indeed made, but the opposite side is with limited by CPU factory, truth is Xiaomi phone is controled by CPU factories, and they got the biggest part of Xiaomi smartphone's profit.
If I can call a deal, I will put all my chips on Huawei, with new self-developed CPU, it will hit a higher.