Dark Irons have a lot of shadow priests, also his parents are both priests; so he looks like the dwarf version of young Anduin, another priest. Adding characterization to a functionally new alliance character to act as a parallel to Anduin's dark angsty present self seems like a no-brainer for this current storyline; not someone making a self-insert.
(Although on a side note, Anduin's angst arc is so unprecedented. Maybe if he actually did dark stuff while he was dominated I'd be narratively earned, but he didn't. He didn't even kill anybody while dominated.)