Some of these ideas are touched upon already, by some of the empires seen in the Omega Octant, Lesser Magellanic Cloud and Triangulum Galaxy... as well as hinted at on the
map of the Milky Way Galaxy itself.
*If you want a very long-lived species, there are the
Helgardians. They each live for about a thousand years, but have a very slow rate of reproduction. In their home dimension, they spent ten thousand years meticulously building up a massive inter-stellar empire; however, the realm they have in 'our' M33 (their home system was mysteriously transplanted across dimensions) is much smaller, since they have 'only' had about 1000 years in which to build it up.
Plus, the
Chlorophons in Omega are sentient trees, which have a lifespan of something in the order of 2200 years!
(No-one knows how long a member of the mysterious Loriyill species lives for, however.)
*On the other hand, if you want social Darwinism gone mad, there are two potential options - the Cloud's
Eneen and Triangulum's
Mallarans.
The Eneen come from a highly radioactive planet, which they evolved to deal with. However, the form of evolution their species took had both benefits and drawbacks; it allows them to both live for 600 years and to reproduce parthogenetically if they cannot (or choose not to) find a partner, but causes at least 10% of all births to be nonviable mutations. Thus, they maintain a draconian eugenics program which would leave your average Federation envoy reeling...
...and they aren't too nice to their subject worlds, either.
The Mallarans used to be a fairly peaceful lot. However, a great plague hit Mallara, leaving the population sterile. So, they developed a machine intelligence called Overmind to counter it... only the AI had other ideas.
Sure, it 'helped'; by taking total control of the population, setting up cloning techniques to propagate the species, then ensuring the loyalty of the citizenry by holding sway over which person's genetic legacy would be passed on to the next 'generation' or not.
However, Overmind soon found its cloning method to have long-term degradation issues; so decided upon culling as much useful genetic material from alien planets as it could, in order to keep the species alive.
Cue a full-on attempt to harvest the gene pools of conquered worlds, and all of the wars that inevitably follow.
*If you want an un-naturally short-lived populace, there are the many hosts of another Triangulum species; the tiny
Arachnids.
These tiny spiders latch on to a victim's cerebral cortex, allowing it complete control over many (but not all; Helgardians and a few others are immune) species. However, doing this wears the host out faster than normal, dramatically shortening their life expectancy.
The Arachnids tried 'farming' their subjugated worlds, allowing their populations to grow before selecting the next batch of hosts. Yet, this allows for no more than a stagnation of their overall numbers. So, they pretty much see the full-on conquest of M33 as the way forward.
Funnily enough, the Mallarans and others have ideas of their own. Cue giant 'splosions in space.
*Also, if you wanted a large, yet potentially fractured power, there is yet another Triangulum option; the
Imperium. In an older era, most of the current Imperium's member species were under the control of the Ancient Ones, under an Old Empire they saw as benevolent. (However, the rest of the known galaxy saw the Old Empire as tyrannical.)
Eventually, the Old Empire collapsed when another species (known as the 'Saviors' outside of the Imperium, and as the 'Usurpers' within) rallied the support of certain subject species and overthrew the old order.
In the wake of this devastating Annihilation War, the two camps went in different ways. The
Frigians, a former major subject species which was the largest contributor to the rebel forces, founded their own Kingdom. Meanwhile, many of the loyalist species formed the Imperium, in the mould of the Old Empire (or as close as they could get to it, perhaps).
The two sides aren't friends.
However, the Imperium itself is not always united. One member, the
Edurian Theocracy, eventually split into a loyalist Edurian realm and an
Order of Ultimate Truth splinter empire, beyond Imperial space.
Plus, in the modern era, the remaining Imperial powers have to contend with an array of upstart 'young empires', such as the Arachnids and Mallarans, and even the
Human Republic I mentioned earleir in the thread.
(I have a copy of
CL23 inbound, as part of a recent order. It's got preview info about the Imperial fleet. I'm looking forward to seeing what they've got so far!)
There are other examples I could call upon. In short, things start to get really interesting once you take a look beyond the Alpha Octant, in my humble view at least.