Thanks to Xiaomi devices' competitive price tags, the developer community around them has grown rapidly over the years. Its size is likely one reason why the Xiaomi Redmi Note 5 (a rebranded version of the Xiaomi Redmi 5 Plus), which was released just last week, has already gotten a flashable TWRP image. It's from XDA Senior Member TingyiChen, who made it available on the XDA forums.
When it comes to custom ROM development, you can't get anywhere without a custom recovery. Flashing ROMs is nearly impossible without one -- it's one of the simplest ways to write to the /system partition where Android operating system files reside -- and they open the door to third-party firmware on the Redmi Note 5/5 Plus. You're no longer stuck with MIUI or any other proprietary Xiaomi software.
There's even better news. The same developer who released working TWRP images for the device also published a mostly functional port of LineageOS 14.1, meaning it's already possible to install custom ROMs on the Redmi Note 5/5 Plus. All it takes is unlocking the bootloader, flashing TWRP, and flashing Lineage through the recovery menu. (If you're looking for tips on how to get started, check out the relevant XDA forum threads.)
To recap, the Redmi Note 5 is the mid-range successor to the Redmi Note 4, and it packs powerful hardware. It's got a 5.9-inch Full HD+ (2160 x 1080) 18:9 screen, a Qualcomm Snapdragon 625 processor, up to 4GB of RAM and 64GB of storage, and a 12MP camera.
Sadly, the phone ships with Android Nougat instead of Android Oreo, and so don't support Project Treble out of the box. There's always a chance, of course, that the developer community will update it with Treble compatibility a la the Xiaomi Redmi Note 4, but only time will tell.