Speaking as a foreigner, I would like to say, how many versions of Koo Gum does Thailand need?! I wonder if it will be better to just do a modern version, like Koo Gum meets Descendants of the Sun kind of thing, just make Kobori live for once or something. haha I don't know. Sorry I was just thinking out loud.
In my opinion, the essence of Kobori is having that "pitiful puppy" expression. He has to have those innocent (almost silly) looking eyes where he's wholeheartedly and steadfastly in love with the N'ek, even when she hates his guts, thus making him look pitiful and you just want to reach out and hug him forever. Of course, conversely, he has to have the X-Factor for the infamous R-Scene, where all hell breaks loose and Kobori is shown as a vulnerable, passionate man after all.
The essence of Aungsumalin on the other hand is having that "beautiful anger" - where outwardly, she's furious (albeit "gracefully" as she is an "honorable Thai lady") but really, she's just mad at herself and you see the beauty in her pain, as you see she's falling in love. She has to come off as stubbornly strong, yet vulnerable.
With that, I can think of some actors who would suit the role well. I think Mew can do the Aungsumalin role. As for Kobori, of course my bias is with James Ji, but I think Toey P. would be suitable as well. I think with a project like this, it's good to cast an underrated actor as the role can really make people shine, if done well. (in addition to the fact that since it's been remade for the Nth time, they'd have big shoes to fill. If you put any of the relatively more "exposed" actors then they'd be criticized no matter how they perform, in my opinion.)
But can we please not have a Koo Gum remake soon? I am still hurting from the Bie x Noona one and the Nadech x Richie one (for different reasons though hahahaha) Maybe in another 5 years or something.