It is also very hard for me to say what could be improved, the first sins is really a classic and very good. my thoughts on the second sins (I have spent about 500h in sins 1 and play mainly against the AI):
A better AI would be great, so more intelligent or a better choice of tactics of the AI, eg that this attacks in shock waves and you know what it does by a description of tactics in the menu before the game starts).
Maybe one or more mods, such as one in which you start with a planet and defend against already a strong opponent who attacks in waves and you slowly expand or a time against this AI must hold.
What I would find good would also be skins, a fourth race.
The new planet overview of all planets is very good, the ships can equip new mods is also very good.
Maybe you can also increase the units generally 10% so you can enjoy the space battles more
The interface is too uniform, difficult to make quick decisions, the icons were better in the first part.
That the researches can't take place on 2 or 3 levels per research and I think, in general too little to research.
If you have an army of ships then I miss an overview of the fleet, as the top left is usually present. Maybe I am too stupid to create a fleet and that is already possible.
It would be cool if the fleet builds up a huge common light blue, tiled or otherwise matching shield and the more ships you have the bigger and more stable the shield becomes, with explosion effects etc.
The Advent Titan could then have an ability that completely drains the energy from the shield of its own units and shoots one or more destructive beams at the enemy.
If I would let my creativity play then it would be cool to be able to choose one of three ship trees per selected race in the military tree, for example the tech could choose between an attack tree, defense tree and a general or matter tree and depending on the explored tree then the ships are more aggressive, defensive, balanced or on matter/skills designed and then have their own look, shoot for example at the attack faction many red laser beams, the defense faction has many blue shields or green healing abilities etc.
But probably it would make the game much more complex. Personally, I would look around in other games and invest a few creative hours, just let what comes to mind and playfully deal with it in thought, maybe find a few ideas, skills, ships, planets, mods, etc..