Not sure how many other people have seen this, but Luxury resources tend to spawn near other Luxuries of the same type. This means that you tend to get massive amounts of a single luxury and you just have to hope that you get a good one (lol 4xRedsang). If you repeat this for everyone else on the map because of game balancing then everyone is basically stuck with a select few resources that you're going to have to use for your system developments (everyone wants Production because Production). In ES1, this would be a 'monopoly' that granted another stat bonus, but because there isn't that kind of mechanic in ES2 you are generally stuck with too many of one resource so in the end you sell it on the market for a steadily decreasing price. This could be pretty easily fixed by messing around with the Resource balancing options or something (I'm not much of a programmer).

Excess Food is one of those things that isn't especially useful outside of spamming manpower ships post-Patriot Pills. It really doesn't do anything besides gain population and it's really boring. We do have the colony vessel food ships as a basis to outsource food, so that's when I started to think about Trade Routes. Trade Routes aren't something I find to be gamebreaking, but I feel like they don't really make much sense. You are trading things for Dust, but what are those 'things' you are trading. In Real life, you would usually be trading stuff for industry and food stuffs. Let's stick with food for now though. Let's say that Trade routes granted only Dust proportional to total food, luxuries and strategics along its route. That simplifies things down from it also giving science that also means you snowball off of having access to even more Dust creation and all that. Any additional food also gets split across all connected systems. This way it helps out your other systems grow quicker and come online sooner. 

If you remove the science component that provides this heavy snowballing effect of basically getting more access to more things to get even more Trade Route bonuses to gain even more science means you don't have to worry about Trade Routes being overpowered. Therefore you can reduce the xp gains required to unlock additional Trade Companies and replace the bonus Trading with 'subsidizing' that increases XP gains for a specific Trading Company.

Now where it ties back in to the Luxury issue is that you expend Strategics to increase your military, but you don't do that with Luxuries. Instead it's a 1 off purchase and then it comes down to whether or not someone else also wants it. So what if instead of your gaining bonus luxuries, luxuries found their way onto the market through Trade Routes? That means that if someone wants to go full Trade then their resources trickle onto the market over time (increasing if player subsidizes a good Trade Route). If the market were to then have a decay on Luxuries, increasing for every System development as if they were being used up, then people would be encouraged to buy en masse from the market so they could then resell when there is demand for it, or if you're the 'monopoly holder' then you'll always be able to dictate when you want to sell all of your merch. 

All of this stuff could be pretty easy to implement since most of it is already in the game.

Enchanteur wrote:

It need a improvement making something like : send excess food from a system with full pop to another system needing food for pop growing.

The improvement do it automatic (no need to manage this).

I created an "Idea" about that just right now :

https://www.games2gether.com/endless-space-2/ideas/835

Problem with just adding an improvement is that it costs precious turn time and only works when you have excess food (no excess food=no benefit which feels really bad). If you add it to an improvement that already exists in the game (Trade Routes) then you remove the boring question of 'Am i actually going to get some use out of this?'