Search under "Trade" and a whole list of threads will come up with discussions and great ideas for a future Commerce & Trade DLC. From what I read, I'd summarize that the ideas fall into 3 major categories:
(1) Economy modeled closer to HOI4 or DWU: Resources with physical locations (planets, ships, stations...), civilian freighters that move said resources within empires and convoys that trade with other empires. Provides for resource hubs, shipping, and logistics chains that are not directly controlled by an empire government, but can be encouraged, protected, and attacked. Those that favor this approach feel it would give greater depth to empire management, warfare, and
provides for a viable reason for pirates. Performance is a major cited concern, but I haven't seen the devs weigh-in on whether this approach is / is not doable... it seems doable in the HOI4 dev diaries... but Stellaris isn't HOI.
(2) Abstracted economy: More like EU4 with abstracted trade routes & trade centers. Simulates a private economy and trade in a simpler fashion, but still provides for extra game depth and strategic choices for players. Those that favor this approach are either concerned about performance and / or don't see option 1 as value added depth nor fun... but the potential tedious need to micromanage a massive commerce simulation.
(3) Hybrid Approach? Some have suggested various hybrid approaches centered on creating trade routes that provide various benefits between empires and have periodically generated trade ships / convoys based on the supposed volume of trade on these routes... convoys that can be protected and raided for tangible losses and gains for the respective parties.
Based on the community discussions and hints by the devs, a commerce & trade DLC is very likely, just a matter of when & which approach Paradox chooses to implement. I've got my favored choice, but I'll save my thoughts. Hope this helps.