In light of user feedback, I will be making some changes to the way co-op works in the campaign. These changes are in no way final, and may be reverted if problems are found with the approach.
Currently, the campaign only affects the host player. While clients connected to the host are allowed to use the hosts's research, they cannot progress themselves. This is because there was no straightforward way for me to carry over 5.0's "parallel progression" system with shared items, as global inventories no longer exist.
To improve the co-op experience, I will be implementing the following changes:
1. Any research the host does will transfer over to all connected clients, even after they disconnect. To prevent gaps in the tech tree, every direct dependency will unlocked as well.
This means is that if you connect to a host and they research a Meltdown, for example, you will permanently get the Meltdown unlocked, along with the Arc, Lancer, etc.
2. Clients will be able to use any research they have, even if the host doesn't have it unlocked.
3. Clients will receive a copy of all the resources the host is producing on the current map, distributed to all their sectors. If they do not have space for the materials anywhere, they won't get any.
4. Clients will be able to research new tech in multiplayer, even if it depends on technology that only the host has. The item cost will be taken from their local inventory - not from the host.
For example, if the host has unlocked the Scorch but the client has not, the client will still be able to research the Arc turret, which normally would require researching the Scorch. In doing so, they will unlock the Scorch as well, to prevent random tech tree gaps.
This system is not particularly intuitive, but it's probably better than the current state of co-op.