I think Horatio Hornblower fits well into a lot of settings.
Star Trek: Pick a war in the setting or two wars separated by a short peace (eg. the Cardassian War and the Borg incursion) Hornblower could also work as a Kirk era "cunning Klingon."
Mass Effect: C. S. Forester's characters could fit on any Systems Alliance or Turian Navy ship.
Star Wars: The late Republic is a good place starting with a peace keeping action that went hot (I think there was one not long before the Naboo Crisis but I can't recall what it was called) and following Hornblower's career through the judicial forces, Republic Navy, and have him retiring a few years after the founding of the Empire. Or you could drop him into Pelleon's orbit as an Imperial officer. Or fit him around one of the Sith or Mandalorian wars that had non-Jedi naval forces involved. Or in the New Republic while they're trying to mop up the remnant. Probably not a fit for the Disney post-RotJ timeline, though.
Honorverse: It's basically already Hornblower in Space, but more direct expies won't ruin anything. They won't help anything either, though.
Learyverse: same situation as the Honorverse except that it's more character driven and loses a lot of its charm if you follow a ship that doesn't have those characters. Sometimes fitting too well makes a potential crossover boring and I think this is an example.
Battletech: A Hornblower Expy is just the man to follow for an Age of War or First Succession War navy story.
Etcetera etcetera...
Another thought is that Footfall can probably be substituted for the backstory of a variety of settings, sometimes a little too seamlessly. The way the Fitph incursion turned out explains the common human diplomacy hat while also allowing them to be as militaristic as the setting demands. The Fithp Incursion didn't result in a double kill because someone figured out Fithp xenopsychology in time, but they also had to fight.
Mass Effect: it would make a good first contact war story pitting the Turians who know Mass Effect technology and warfare using it a lot better against a pair of species that developed a space war doctrine around a battle fought before either had discovered eezo. By the time the Reapers show up it's a stomp fic, though. They rely on directing the technological evolution of their targets and orbit to surface armor killing particle beams and bomb pumped lasers are not what they were planning on facing. Reaper Stomps are popular, though, and the Systems Alliance-Citadel Council dynamic in that era could be fascinating to explore. They're uppity newcomers, but they're uppity newcomers with a client race and disruptive military technologies who probably humiliated the Turians pretty badly even if they were losing in attritional terms before the Asari negotiated a peace. Also, "fast Elcor with multiple prehensile noses" are probably unpleasant to face in ground combat in a way that would make some people regret not finding a solution to the Krogan problem that kept them on the Citadel's side.
Star Trek: the backstory of Star Trek is hazy and doesn't very much matter. You probably lose the augments, but they only actually come up a few times. You also lose the post-apocalyptic horror but that only comes up in Q's set dressing. These are stories that don't need to be retold in a Footfall/Star Trek fusion because they were already told in Star Trek so I count them no loss. The Vulcans didn't initiate first contact OTL for sublight sleeper ships like the Botany Bay being sent out, but seeing humans and fithp peacefully resolve a very messy first contact war on their own might result in earlier Vulcan contact or it might not. I'm not sure why they didn't make contact with a species sending out sublight colony ships OTL so maybe whatever that reason was still holds. The Prime Directive actually makes sense in this fusion as the Humans and Fithp suffered a first first contact about as bad as it could possibly be without one species winding up exterminated or permanently enslaved followed by a friendly second first contact with the Vulcans. Humanity doesn't want to interfere in the natural development of other species because their natural development was cut off by the Fithp. This never made sense for a humanity that was lifted out of the post-atomic horror by the Vulcans as their first contact. I'm not sure this stays interesting once they're just another Federation member species, though. I think this story has to be set between the Vulcan first contact and the formation of the Federation or be about a Fi integrating with a non-Fi crew in whatever era ships become large enough that a Fi isn't going to obstruct corridors sized for Humans, Vulcans, Andorians, and Tellarites wherever he or she goes. Fish out of water isn't my cup of tea, but I know it's some peoples'.
Battletech: The early history is obviously very different and there are some pre-KF colony expeditions, but by the KF drive expansion wave things can have converged. The Fi probably colonize different worlds from the Humans, but after the Outer Reaches Rebellion they are part of the process of amalgamation into major states. The development of the Battlemech is flipped, though. Instead of starting with Mackie the tank killer it starts with ultralights or primitive protomechs or BA to let human infantry fight on the same battlefield as Fi infantry. By the Star League things have mostly converged again, but having two species allows for dynamics like the Combine being two quasi-separate racial supremacist nations (as the Nazis allied with the Japanese) because the Kurita are the destined rulers of Humanity and there's no reason there can't be some Fi family with similar aspirations. or Amaris getting to add genocidal racist to his long list of vices. Infantry is a lot more respectable and heavier and while Fi social structures are different they have the same capacity for dickery as humanity and while OTL Cameron's obsession with imposing feudalism on everyone reinforced the Cult of the Mechwarrior it still happens without that cult which might still emerge with the development of heavier mechs. There are some dynastic restrictions on which ruling families can be replaced with Fi. I think the idea of the Combine being a twin nation split on species lines is promising for the setting with two coordinators one Kurita and one Fikurita both claiming the right to rule their species with whichever is more senior at the time being the senior ruler of the Combine and every multi-species world conquered having the minority population forcibly transferred to its species worlds so the twin dragons can each have ethnically pure worlds. I don't think the Mariks or Cappies are involved in any dynastic links I care about so they're good candidates for replacement with Fi dynasties. I think the Camerons have to be human and they have dynastic links with the Davions who have dynastic links with the Kuritas and Steiners and the Kuritas have dynastic links with the Sorensens and I get the impression from the wiki that the Sorensens and Magnussons are both descended from the ruling family of the Principality of Rasalhague. But as I said I think a good solution to the Kurita destiny in a two species setting is to link them up with a Fi dynasty with the same delusion of rightful rule.
Schlock Mercenary: This is another example of a setting that fits too well to have any interest whatsoever. The elephants have an extra trunk and there is some mention of Alpha Centauri being more prominent in the UNS as their homeworld. That's really it. They fit so well you could easily miss the crossover if you weren't paying attention. There are probably a number of settings like this, perhaps more than are actually interesting. Star Trek was almost one.
Warhmmer 40k: I don't know the setting well, but I think this is interesting for the ways in which it thematically doesn't fit. What if humanity wasn't alone against the horrors of the setting? There are lots and lots of Xenos that definitely need purging, but there are also these Xenos that have been a human client species since before humanity had interstellar space flight and they're people. A little odd, but definitely not eldritch monstrosities. There is definitely a lot of purging going on, but the enemies are named because there's the possibility that they might encounter other aliens that they can ally with (or at least use as cannon fodder) against the forces of Chaos and the Orks and Necrons and Eldar. And there may be some species they exterminated OTL that they do uplift as cannon fodder TTL. Narratively it probably changes almost nothing, but thematically it's a substantial shift.
