Tristan VS Isolde implies that Tristan will defeat her and even as he is now his body refuses to do it. Kinda hard to imagine Isolde fighting earnestly against him too. This is too annoying of a MU. I could see him fighting Chion, but not her. As for Percival VS Nasiens, I don't see how the fight would be difficult at all. They are all in this together. The only problematic MU among the team would be anyone VS Anne because of Interrogator.
I think Nasiens mostly has to show his new abilities as the next Fairy King in this arc. That's why I believe Tristan is a good opponent for him since he is technically an antagonist in this arc and is much stronger. By defeating Nasiens, Tristan cements himself as a foe to the others as well.
Tristan VS Isolde doesn't need to be the first MU, it can be the last one before the interference. I placed it first because it's the one I'm most certain of owed to what he said when leaving and their deep personal connection( Nakaba used personal connections to add drama to some of the Vaizel 1 and 2 tournaments, I don't see him changing strategies). And the fact that he can't fight her seems relevant to solving his memory/confusion issue. I see Gowther necessary to and involved here for dealing with Berto, not saving Tristan. Up till this arc you were all about the teens sorting out their messes together, now you want a senior hero to solve something for them?! It doesn't lead to any growth for the heroes and gives no serious grounds for Tristan remaining antagonistic( which is unnecessary too, the Perils have already had their danger factor upgraded, much like the New Gen HKs way back when, which is why it's good to have Gowther, Howzer, Pelliot around, to provide more experienced support).
EDIT: I forgot to add, much like Anne would be trouble thanks to Interrogator( which should be fun), we need to see some development, fighting-wise, for Isolde too; and some growth going hand in hand with her epiphany during the fight with MacDuff. Before that she claimed she was fine with just serving him as an underling/bodyguard, but when she was about to be killed, she had the epiphany that she, actually, wants to be his lover/life-partner. So we need to see her shaking off her doubts and tendency to just stand there and take the abuse coming towards her and give Tristan a good shake off. Both Elizabeth and Gelda wound up willing to give rude awakenings to their lovers, I don't see Nakaba passing off the chance to do this with Isolde too.
I don't see why the parental drama would be relevant to Nasiens VS Tristan, Nakaba tends to use direct connections between the contestants to provide drama during MUs. There will be impromptu conflicts here and there, like the random fight between "Matrona" and Griamore, or the Mel & Ban duo facing off the blue demons, but most of the MUs he creates go hand in hand with personal well established conflicts, smaller or greater. Nasiens has no genuine beef with any of their overall teammates. But has a change upcoming and apparently teased more and more, and that's what's relevant to them. Both showing off their magical growth or relevance( like Diane showing off Mother Catastrophe and King finally properly using Chastiefol) and defining their identity( at some point I do believe just one gender will be chosen, why not now?)
Tristan has been worrying his parents because he's been missing for a while, he currently has memory issues while Nasiens, who thought his parents had abandoned him, learned the truth recently, after spending 2 years with them. I think he was even the one who said that his parents were worried when they met at Annwfyn.
This is a reach, sorry, but it is a reach. Chion also worried his parents, and Tristan's( to my great annoyance) don't look as worried as actual parents would be. Howzer, who's the closest thing Donny has to a father( well, he technically took up the parental mantle from his sister and BiL) was also shown to be worried and even came along to give some support. Nasiens already sorted out their personal parental drama, Tristan did have a resolution during the fight with the CKs, to accept himself as Nephilim and take on the duty to make his parents(mainly Mel) proud with the gifts he has, both demonic and celestial, it's why he felt entitled to the Dragon Handle once Percy threw the towel. For that reason, even a Tristan VS Percival makes more sense than Tristan VS Nasiens.
. But I am not too sure if he'll fight Percival or not. What I find more likely is that this arc will fan the flames for their sibling rivalry. Percival defeats Turret and another KoA or one of Percival Platoon's members defeats the other. I thought about placing Rothes instead of Isolde as Gawain's first match, but it was mirroring Percival's path too much. Gawain beating the shit out of him also works.
