Xiaomi has released an AOSP build of Android 5.0 Lollipop for the Xiaomi Mi 2/2S. The build lacks all the features of MIUI and is completely stock Android.

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The ROM does not have any major issues but there are some bugs, which might hamper everyday usage. Also, being an unofficial ROM, you will have to flash it manually and it won't be pushed as an OTA.
It's unknown whether Xiaomi will release an official Lollipop build with MIUI for this device later but if you want to give this a try then head over to the source link.