@artch Although you've created the Seasonal world as a competition, I believe it has the potential to be so much more. It's an excellent sandbox to test out new and innovative things, and give you full liberty to try some really creative explorations outside of the realm of regular screeps gameplay.
As far as new player goals are concerned, I think seasonal is a wonderful format to get people started in a friendlier and safer environment without totally messing up the core gameplay of regular screeps.
Alternatively, you could potentially create an entirely separate fully co-op MMO world, but this is a significantly larger undertaking.
We have to only stick to ideas that imply that only a few people get significant reward.
As far as paying for season in general, I'd caution leaning too heavily towards the feast-or-famine reward model, as it strongly disincentivizes participation from all those but the most elite, which isn't good for the long term health of the game.
For a global co-op season, I think you might be underestimating the willingness to pay for access keys to a fun and interesting coding challenge as its own intrinsic reward, again, particularly among new players. Doing a marketing push on coding school platforms could potentially do really well.
I'd also suggest that limited edition cosmetics are always an enticing reward. Giving limited rewards as CPU tokens could additionally go a really long way towards converting non-paying players into lifetime subscribers. $10 in access keys as an entry fee, $3.50 in CPU tokens as a reward, plus a limited edition non-transferrable cosmetic. Choosing to scale the reward up or down based on time to goal completion is entirely optional, but a fun incentive to put up for players.