To start with, while I personally like the established Races in Galactic Civilisations and I would like to see them return, the idea of adding more Races is inevitably going to be popular.
In a way, I think soliciting people's ideas for Races is looking at the question the wrong way - sure, the result is much more likely to be higher quality by filtering out the really stupid ideas people can come up with, but if the idea is to explode the galaxy wide open and really unleash creativity, the most effective way is to give players modelling and rendering tools to develop their own Races.
Imagine opening up the game, and clicking on the "Create Race" button.
The display then gives a blank slate - the player selects the kind of lifeform they want this Race to be, whether carbon-based, synthetic, gaseous, aquatic, silicon etc. and then the display places the holotype definer above a platform (stood, spread out, hovering, whatever) with measurement reference points, and players use a combination of push and pull tools, additional body parts, texture and render tools to shape the amorphous blob into their vision for the holotype. Additionally, there could be templates that offer generic starting points - an aquatic Race might have generic templates based on fish, squid, crustaceans etc. for example. Animations for limb movements and emotive behaviours can also be defined as part of this process.
This holotype now can now have variants designed - if the holotype is blue skinned, then variants might be cyan to purple as a simplistic example - and clothing and adornments could be added to create individual members of the Race. These variants can be used to then generate citizens in association with a naming structure/list. The rest of the process of Race creation then proceeds within the standards GalCiv has developed.
The biggest advantages are that any future Races all are coherent in art style (with Galactic Civilisations 2 and 3, what happens is you end up with clashing art styles as people import photos and videos from different media types) and individual Citizens can be automatically generated even for custom Races - the new focus on Citizens in particular makes this proposed system extremely advantageous - and that it inherently offers the ability to have all Races display in videos; while GC3 supports videos as part of custom Races, this is extremely difficult and complex for players to make, and probably 99%+ of Races (at least, ones that don't reuse GC3 video assets) use static images only.
It is ultimately more work, as it is effectively making a 3D graphics program as part of a game, but the result is fantastic for players and would make adding more Races easier for the Devs too.