You can mod Overwatch's Mercy into XCOM 2: War of the Chosen

The [WotC] Overwatch Mercy Squadmate cosmetic mod adds a very convincing Mercy facsimile to XCOM 2: War of the Chosen with custom colours and wing variants. In the name of authenticity, "this mod adds Switzerland; Mercy's country of origin."

Check the description section of the Steam Workshop page linked above for a choice of downloads and some recommended mods that help Mercy to work smoothly. Missing Packages Fix + Resource sounds like an essential one, while the Mercy Voice Pack ought to be a fun addition.

It's a purely cosmetic mod, so don't expect the mod to add any of Mercy's abilities into the game. It looks good though. Check out a few of the colour variants below.

Based in Bath with the UK team, Tom loves strategy games, action RPGs, hack ‘n slash games, digital card games… basically anything that he can fit on a hard drive. His final boss form is Deckard Cain.