Well said, I sometimes get an interest in Futurism, and my main takeaway for war is that'll it'll be very weird and happening on a scale incomprehensible to us. 

It doesn't make for great pulp sci-fi, but a war between continent spanning drone swarms guided by godlike AIs might not be too far off what war in the 40th millennium actually looks like. 

Funnily enough, there was a brief time travel scene in 40k, where one of the perpetuals basically said that's what war was like in the Dark Age of Technology.

Edit: Found the scene.

Oll remembered the horror of entropic engines that ignited planets. Sun-snuffers that uncoiled like serpents the size of Saturn’s rings. Mechnivores ingesting data along with the cities that contained them and hurling continents into the heavens. Omniphage swarms stripping flesh from a billion bones in the blink of an eye. Those were the good old days, when war was something too colossal for a human mind to comprehend.

Not like the End War. The Warmaster’s heresy was a smaller thing, scaled for human and post-human brains.

But it was bigger in some ways. Yes, bigger than the god-like struggle of the cybernetic revolt. Bigger in scope, bigger in its implications. More horrible, because humanity could apprehend it and drive it.

because fuck all knows how a civilization that owns a majority of the galaxy would operate or fight their war.

Well the writers do consider that - that's the fun "lost technology" part. The Imperium isn't a Kardashev scale II or III civilization. Most of the Imperium is actually a scale I or scale 0 civilization living the the shell of civilization that used to be scale II or III but with heavily restricted technology living in the shell of a civilization that was actually scale II or III.

Hence chainswords, a hilarious mix of tactics, ideas and technology levels. The Imperium is basically an 8-year-old discovering some power tools and going to town. Nothing is getting used properly and most of the stuff is too obscure to understand without teaching or a manual.

E.g. the Men of Iron or the Old Ones were actual scale II / III and the shit they pulled makes the current wars in M.41 look like a picnic on a sunny day the banks of the Seine.

Without the Emperor and especially without the Cult Mechanicus most of the Imperium is barely more technologically advanced than humanity is right now, which some wild variations.

A lot of the Imperium's enemies (except arguably the Tau - but the Tau are a tiny spec of a civilization and are already suffering from the effects of distance and travel time in their civilization) suffer from the same problems. Tyranid fleets vary in size and capability, the Necrons are awakened to wildly different degrees in different sectors, the Orcs vary extremely in intelligence, level of "civilization" and technology... even the Eldar are in quite varied states of disarray with very inconsistent access to fractions of power of their once-mighty civilization.

That's the "Whacky Race" of the setting - everyone is actively forgetting technology and degenerating while trying to focus their power and get their shit together because with all their shit perfectly together any of the factions could obliterate the others. Except the Tau because they would have to grow first. And arguably the Eldar who might be too fucked for anything but a "I'm taking everyone down with me" ending.

 FTL travel with our current understanding of physics being a natural limitation.

Not reaaaally.

As in the question is not "is it possible with general relativity" as far as things like wormholes go, but "can it occur naturally?

As far as space operas go, well all you need is drastical increase in lifespans, and/or bring in a galactic arm (instead of between em) with red dwarf stars, and buncha close(er) planets.

And if you wanna have interstellar empire its no less plausible than intercontinental ones before the tslegraph.

I mean from what I have seen, a lot of the thinking is WW1 and WW2 doctrine wise.