I've found the term 'contrast' fascinating lately, not just because I've come across plenty when making MUs myself, but also (and I'm aware how much of a hypocrite I'm going to sound for this stance given one wanted Xeno MU of mine is Melia vs Magi Aladdin)... studying Shulk vs Lightning and still finding it mid made me realize that some characters can have too much going on in their narratives to make their MUs viable, and it creates this sort of chronological dissonance because of it. Sometimes we ignore that disconnect and call it a 'contrast' regardless.
Take the above Shulk vs Lightning comparison as an example: sure, there are things that you could construe as a relation, but they're misaligned at the best of times. Compare the Bhunivelze Hope Estheim builds to A) the one that becomes a god and the final boss of LR, and B) Zanza, what happens with Klaus, and Z (and Origin, for that matter), for example - the closest analogue Bhuni' has to the 'mad scientist' Klaus was is mentioned off-screen in a databook somewhere, while he bodyjacks Hope to make him more or less the Alvis/Alpha in this situation. Now you might be thinking 'the fact that he doesn't have as much to do with Z's behaviour doesn't really matter, because the thing that is important is Bhuni' and Zanza wanting to recreate worlds and universes' - but here's the thing: that relation on Lightning's end comes towards the end of the XIII trilogy, not towards the end of the first game like what happened with Shulk and Zanza.
To further impress this point upon people, keep in mind what actually happens in XIII-1 (the whole Cocoon war where people were starting to forget why they were fighting beyond not wanting to let Barthandelus send them to the Purge (see also the comparisons to be made between Homecomings and the L'cie turning to crystal after completing their Focus)). Several times throughout XIII-1, I found myself saying aloud 'in Lightning, I see more Noah than Shulk' (y'know, if Noah was Alexathymic and wanted to treat everyone else like they were stupid?). And even if you want to say that the Avatar's from FR and Lightning juggling positions as Bhunivelze's saviour and Etro's replacement could be considered such a similar contrast (...that's not a term I've used in a while)... is it really that? Or is it missing the point of these characters? Shulk has unabashedly rejected Godhood on basically all accounts (I'm aware that the queens building Origin and treating Klaus with kid gloves is a bit of a hard sell, and I'll file that under rewrite(s) for FR and (to a lesser extent) XCXDE), whereas Lightning in LR more or less lets the manipulations of Bhuni' happen because she wants Serah back (side note: how Bhunivelze became a biblical deity when Hope came up with the name on the spot is genuinely never explained, at least to my knowledge). Could you argue that being an Avatar of Origin in FR is a form of Godhood? Honestly, I'd say no, just simply on a count that Z ran roughshot anyway, and to the players, that 1000-year timeskip sort of stops mattering in the end when both universes returned to a non-Endless Now reality.
So what's my point after all this, and responding to a post about alternate Death Note MUs? Well, one is of format, and if these would warrant a death battle to begin with - to wit I say the LightLelouch suggestion would probably suit a sort of 'could Batman solve the Kira case' AU (I'm aware that's one of Seth's old video, and if this means attributing that idea to Clyde and the others that Seth has abused, I'm completely OK with that), as at the end of the day, Light is just a guy in combat prowess, while Lelouch has mechs and shit (which is why Columbo fits, because the 'fight' was never about physical ability, but cognitive capacity). But my other point is realizing the opportunity cost (in time, largely) of the people who have to verify the validity of MUs like this. Is it truly that enticing to both the more and less involved in communities and debates like this when they have to sit down and binge a series like the XIII trilogy, only to go 'hang on, X relation should have happened by this point' or 'Y circumstance has nothing to do with these characters'? That's not to say you can do nothing with that fact (any N vs Caius fans in chat?), quite the opposite, and it's what makes those MUs more lucrative. From the POV of a Xenoblade fan, I think that the relations Lightning has come off as all over the place, but if I was a XIII fan in an alternate timeline, and I went to binge the numbered Xenoblade trilogy... yeah, I'd see the comparison, but what if I come out the other end thinking 1 was rushed in it's ideas? What if I end up annoyed that even by the end of that game, my comfort character loses (I'm talking from the POV of that alternate timeline where I liked Lightning for some reason, for me now, Shulk winning would actually be pretty based)?
Just something to chew on.