You can have Mercy from Overwatch as part of your squad in XCOM 2, even if she doesn’t actually resurrect your fallen comrades.
This is obviously done through the power of modding. In this case, the mod in question is simply called Overwatch Mercy Squadmate, and you’ll need the XCOM 2 expansion War of the Chosen installed. It adds a really good 3D model of Mercy that lets you take her to missions as a squad member.
The creator even got her wings and hairdo correctly, and you can change the colours of her suit and pretend you’re looking at a new skin in Overwatch. Among other smaller changes, the mod adds Switzerland as a selectable country of origin for soldiers, which is where Mercy is from.
Although the mod doesn’t add the ultimate ability to resurrect, you can just stock her with medkits and stabilisers and pretend she’s resurrecting people.
You can download the mod from this Steam Workshop link (via PCGamer). There’s a list of recommended mods you should also have installed, including one that adds the the various voice lines from Overwatch.