By choosing to re-read Pratchett's "Night Watch" instead of picking up the Brick.
But I can't write a fusion with this because it's just too easy. I mean, to start with, Night Watch basically is a Les Miserables fusion already. I'd been wondering at what point I became aware that Les Mis has had a small-but-active fandom all these years, because I've talked to a few other people who I thought would know and find them completely shocked, and now I'm suddenly thinking that I'm pretty sure I read this book back when it came out, then went and read all the Vimes/Vetinari, and when that ran out [far too quickly], I went and looked for Javert/Valjean.
Not that the ships are at all the same, but -- well. Vimes on the barricade has Grantaire's alcoholic cynicism and Enjolras's obsessive love of country and charismatic leadership and Javert's copper's ethic and tarnished but unshakeable faith in the primacy of Law and uncontrolled urge to sass authority and perfect dramatic timing and Valjean's sullen anger and paternal benevolence and socially awkward class mobility and expansive compassion and Bahorel's boiling violence and Marius's loving wife and happy ending and etc, all mixed up together into everything that is fandom catnip for me and kicking ass with competence. And frankly that explains so much of how I've been reacting to Les Mis fandom. And I got a bunch more of the direct references this time and I kind of want to read the Brick just to find out how many more there are.
Anyway, the problem here is that there are too many possibilities and I can't pick just one. (I know, I had the same problem with Highlander and just went with all of them, but I can't do that every time.)
There's the one I played with first, with minimal direct crossover elements, where Elderly!Steampunk!Combeferre is playing with a time machine and accidentally sends one of his old friends back to a few days before the night of the barricades (perhaps badass grown-up Gavroche?) And he gets adopted by Musichetta-the-seamstress-who-is-
But then it is also SO TEMPTING to just write Les Amis as one of the groups manning the barricades of the Republic of Treacle Mine Road. (Would they be students at Unseen University in this version? Why does that image simultaneously terrify and enthrall me?) Bahorel would get his eggs! :D So much other amazing possibility! Although I think want I most want out of this version is Enjolras and bb!Vetinari being VERY INTENSE at each other about politics in Snapcase's Ankh-Morpork, and then not having any sex at all. Ever. (I actually have had all sorts of THOUGHTS about the political philosophies underlying much of Discworld for a long time, it kind of needs an Enjolras to be intensely republican about it, so much.) Meanwhile, wizzard!Grantaire tries to pick up Young Vimes at a tavern, but ends up just getting him very drunk while listening to him rant about his love for his bitch of a City, and begins to suspect he has a Type.1
But of course there's also the swirly portal type of crossover, and hey, this book comes with a swirly portal built in! Sure, in canon it's through time and space rather than between universes, but it's also in the Library of Unseen University, which is connected by L-space to all other libraries everywhere in the multiverse, so it's not entirely impossible that the Librarian might have picked up a certain brick-like tome and attempted to use it to stabilize something and accidentally made a connection?
Maybe Vimes fell through to the Paris insurrection instead of the Ankh-Morpork one! And pointed out to the barricade boys that they were DOING IT WRONG. And then made everything better. And then Combeferre gets to do steampunk mad science to send him back.
Or maybe Javert jumped into the Seine but landed on the Ankh! And bounced, of course. Although I think in this version I'd want him to fall into modern-era Ankh Morporkh instead of barricades-era, because I want him to end up annoying Vimes so much that he gets put on Moist von Lipwig's security detail, because that would be hilarifying and full of sass. And gets adopted by Lady Sybil, which he would handle both much better and far, far worse than Vimes did. (Or hell you could do this one without the swirly portals and just write Vimes as Javert's superior in the Paris Police, which would be equally hilarious really.)
There, I have written almost as much in bunnies for this as I did in actual fic for the others. :P Now why hasn't any of this been written yet? *sighs*
1This is probably The One I Am Most Likely To Actually Turn Into Something I Could Post To AO3, but we're back to actually needing to learn about Enjolras's political philosophy. And figuring out how the city government of Ankh-Morpork actually works.2
2 Screw it. Fine:
( May 25th, in the second year of the patricianship of Lord Snapcase )
(..also can I just note for those of you trying to figure out Javert's timeline that Vimes' timeline is impossible? The Ankh-Morpork novels contradict each other incessantly, and that's even without the parts that involve time travel.)