I've read something about new potential bitmap customization features in the next releases but I'm not sure if this also applies to Leader Portraits. Working with the provided GUI XMLs, Is there currently a way to pull a custom picture into a custom major faction ? or simply into an existing one, as a replacement ?

Also, is there any game functionality in the Amplitude product roadmap intended for offering more customization in the main diplomacy screen so that, in case of a game with, let's say, three custom Terran factions, i'm not always talking to "myself" ? A simple optional removal of those holographic animations would be ..something at least (text could be enough).

I'm looking forward to start the implementation of mods inspired to Sci/Fi market content where multimedia customization is a key factor. There are of course alternatives like Stellaris or Gal Civ III offering such enhancements that have allowed the creation of great works based for example on Star Trek's world and creating a solid playerbase. 

There's no need, in my opinion, to push roadmaps over the normal commercial standards but basic XML graphics manipulation should be provided asap (custom images, custom static maps, animations, models..) or modders won't be able to give ES2 the longevity it deserves.