I think the reason fallen anakin makes the most sense, or Hayden vader, is because of their focus on revenge of the sith. When you think of kenobi in that movie who is the obvious opposite? Maul? He's not even relevant in the movie. Dooku? His head is rolling around a space ship for most of the movie. Grevious? He fought Kenobi but it is tough to call that a legendary fight. The o ly thi g that fits is the duel between kenobi and anakin. I get people are butt hurt about factions and "it wouldn't make sense if he was separatist" or some nonsense like that, but quite honestly when has this game ever gone by canon or making sense? They base factions off of balance. They generally choose characters that tend to be opposites.
You can argue that Jesi Master Luke and Sith Eternal never fought so it doesn't make sense, but CG was just capitalizing on the new trilogies and knew more people would pay for a luke and Palpatine than another kylo or another rey to counter luke or see. Same thing here, while some niche fans want dooku or maul or grevious, Hayden vader will make them more money. That is the obvious choice.