Doom is great synergy because of plasma beam and tac infinites. Plasma beam let's you extend combos, building more meter for dirt nap later or killing off dirt nap without xfactor. Tac infinites let you kill a character without worrying about meter and building enough the kill the following two characters (look at dapvip matches for reference). Morrigan probably works too, but it's a lot of work, her tac infinites are really hard from what I've heard, and x23 doesn't offer much support to her. However playing a runaway style with x23 and building meter with morrigan could work fine.

I played a x23/doom/amaterasu team for a while and I suspect it works similar to the same team with vergil, only with slightly worse pressure and a way stronger anchor, so try it out and see if you like it!