While I clearly would have prefered a whole new character instead of "just" Arjuna Alter, what bothers me the most is that DW didn't use this opportunity to give Karna this awesome form https://www.zerochan.net/760449, known as Botsu Karna (or Death Karna) in Japan, which is basically an Alter form of the Son of the Sun God and would also have changed his class from Lancer to Rider.
I mean, his Super/Burning Three Gods form doen't look bad per se, but goddamn, it would have been sweet to have a an epic "Berserker Arjuna Alter vs Rider Karna Alter" battle.
I think it's like Russia all over again, instead of utilizing the rich roster the country has to offer like Peter the Great, Rurik, Olga of Kiev, or any of the Bogatyrs, they've opted for using characters the pop culture(or in this case, FGO's own fanbase) is already familiar with, namely Karna and Arjuna.
Could've gone for Kalki, Indrajit or Krishna for other characters closer related to the theme of the Lostbelt, but nope let's stick to the ones they already know the fujoshis are gonna whale for.
