oooh im so glad theres new questions! in an old post you mentioned peach was originally gonna sign her name on the contract to help luigi. would bowser have let luigi go? was there originally gonna be a different ending?
There wasn't a different ending planned because I'm not a writer who plans out everything before I start. Eg, when I posted Day 20 I didn't have more than a few short interactions written up for Day 30 (though I had more of Day 21, while Day 27 was barely a vague idea of a post-ending bonus scene). It took the first trailer - not the one the bowuigi crowd lost their minds over - for me to get the bare bones of the rest of Days finally hashed out. A lot of plot details only make themselves known as I'm writing.
I don't think there ever would've been an ending that didn't involve the contract breaking and Luigi leaving temporarily. But that early into writing (with only Day 1 written), I had no idea how the contract would end up being broken. As soon as Mario showed up in Day 20 it was obvious to me that he'd have to be the one to do most of the legwork, but before that there was a chance it could've been Luigi somehow - or even Bowser himself!
I think I decided against Peach signing her name because Luigi and Bowser's relationship wasn't at a point where it could withstand that. Yes, by then Bowser's aware that he likes Luigi and would be pretty interested in something more intimate and long-term, and yes, Luigi's starting to subconsciously fall for Bowser and find comfort within the castle, however, it's still incredibly fragile.
If given the strings-free choice of Peach or Luigi, Day ~20!Bowser is picking Peach every time*. It's the week of living with both of them (and forcibly being disallowed from flirting with Peach) that helps Luigi get the final leg up there. Luigi is engaged with castle stuff and reciprocating (even if only unintentionally) and Peach... isn't. Don't underestimate how attractive being wanted can be.
*(ok, he's probably going to make at least one play for both, but even that will be a Peach and-Luigi rather than Peach-and-Luigi, if that makes sense, and he'll pivot to just focusing on Peach pretty quick, especially if his flirting towards Luigi is now getting him zapped)
And on Luigi's side, he'd just push down familiar jealousy and disappointment. He's used to being overlooked in favour of someone better. He's not going to fight for feelings he doesn't even know he has.
Getting much more speculative now. If Peach had signed her name then that would've freed up Luigi to find the contract breaker and that side of the plot would've been wrapped up way quicker (Mario&Luigi working together get things done faster than Mario having to recruit a party of misfits). Which means that the final battle with the Capods would've been more of a focus (which means I would've had to have written more actual fight scenes and things were taking long enough as it was!) and I probably would've had the characters converge again during that. Have Bowser and his army show up for the final fight (kinda like him showing up in Day 20, but on a bigger scale).
And in order to make the endgame Bowser/Luigi stick, there would have to be... something happen there. I was pretty determined to stay with Luigi's pov the entire time (yes, Day 16 gave us some Bowser insight, but that was posted post-ending and I tend to prefer sticking with one pov and the restrictions involved with that) so we wouldn't have known what had been going on with Bowser while Luigi was adventuring.
Though, it could be that the act of Luigi asking Bowser enough questions to fill the audience in reminds Bowser that 'oh yeah, I like this guy', with Luigi having a similar 'oh thank goodness, I feel safer with Bowser around... wait, since when has that been the case?!' and that's when Bowser would make his move.
It would probably feel more rebound-y than (I'm hoping!) Day 30 came across, given that there's been a more recent, more successful attempt at Peach (~5 days of being magically married is the closest Bowser's ever gotten - even if it's purely to save a friend, it's still voluntary on her end and that's a win for him). But a relationship start is a relationship start, so that might've been it!
Tl;dr - Yes, Bowser would've let Luigi go and that would've required a different ending, but I didn't have anything planned by that point.