This is probably one of the most requested Xenoblade Chronicles characters for Smash Bros. Ultimate. Even though transformation characters are pretty much gone now, there are still a few ways that this combination of light and fire blades could work.
First off, there’s always the option of making Mythra and Pyra a transformation character. Pokemon Trainer still exists and even though the fighters aren’t technically transforming, it’s still the same concept.
If you wanted them to be individual fighters, then you could have one of them be an echo. That way you can keep all the Mythra and Pyra fans happy. Then give them their special attacks from XC2 and make their level four special their final smashes and you’re good to go.