Yes, 'Elder Races' would be a very nice feature to have either as playable races or as 'wandering monsters' in the galaxy.
I think that a good population AI might make 'Elder Races' doable as playable races. You'd give the Elder Race population a very low growth rate and a very high resistance to the sorts of adventures that younger races normally tolerate. Invasion and conquest, for example. Since an Elder Race starts with a presumed very high level of technology and culture they are all highly individualistic and not willing to 'sacrifice' themselves for some petty war. Normally, if you attack a race that hasn't attacked you, your population would shut you down. They are all 'artists' or 'scientists' or 'scholars' after all and not interested in being shipped off to fight a war. So, you couldn't rush out and attack anyone and everyone and you couldn't necessarily colonize a lot of planets beyond the first few turns because of the very slow growth of your population. They'd probably not want to even have trade treaties with the younger races because they wouldn't have anything of interest technology wise, and their culture would seem seedy or outright vulgar and barbaric to the advanced Elders. On the other hand, you
would have lots of technology goodies to work with from the very first turn and you could possibly use those to manipulate or bribe younger races to do your dirty work for you. Kinda like the Babylon 5 universe.
