With the recent addition of the SoI system, it has become way more important to actually split up regions within an alliance, even if they are only prepped by 1 central nation. This is so nations within an alliance can better support eachother in sieges. This means over time (we likely wont see it untill this test phase of the system is over, but im certain this would happen if this was a permanent change), theaters would become rarer and rarer, to the point their existance is basically not needed anymore. This is why I suggest changing the way theaters work, it can be anything but I want to bring up 2 different ideas I had:

1. You get part of the materials but only need to hold 2/3 of a theater.
This will likely be a lot easier and more convenient for alliances and still rewards a nation that puts the effort in to prep multiple regions for themselves, rewarding them with maybe 50% of the theaters payout.

2. An alliance needs to fully control 100% of the theater and each nation gets specific % of the payout of the theater (based on how many regions of the theater they hold, it should still add up to less than 100% to reward one single nation holding an entire theater). An example for this could be the following. Canada, Ironwood and Rome hold the maritime silk road theater, each nation holds 1 region. They need to all be allied to every nation controlling a region in this theater (canada needs to ally rome and ironwood, rome needs to ally canada and ironwood and ironwood needs to ally rome and canada). Only if some "triangle" of allied nations like this hold the theater the reward will be payed out, but each nation only gets like 20-25% of the payout instead of an even split of 1/3. If rome has allied both ironwood and canada, but canada and ironwood are not allied to eachother NO ONE gets a pay out for the theater.

I think this would make theaters still useful with the SoI update in the future and yes I am aware siberium currently holds the maritime silk road theater, although i dont think this will last very long if this system is implemented permanently. If you come up with other ideas let me know, these were the only 2 I can think of, but a change is likely needed.