The phrase that started this whole project was "Exploration as progression." Many games give the player access to new areas as they get farther along, but that's "access to exploration as a progression reward", and I want the act of exploring or finding new things to be the progress that gives you the reward. It's not immediately obvious how to achieve that, however. Sure, you can take the percentage of the map you've explored, or, in the case of an unlimited map, a measure of how much land or how many places the player has been and give rewards based on that, but that linear system fails to capture the essence of exploration to me, which is the joy of happening upon unique or unusual things.
How, then, do I put rewards in place that do reflect the uniqueness of what has been discovered? I've mentioned in a previous post that beauty makes exploration satisfying in itself, and the player can be the judge of how interesting what they have found is; but that's only one kind of exploration, that of seeing new places. "Exploration" to me also includes doing new things, pushing the limits, the "what happens if I..." or the "do you think I can...", and that's not to mention using the other four senses besides sight in a new place.
It would be difficult for the generation algorithm to recognize features that a human might consider unique, unusual, or otherwise interesting. Even if some other post-generation algorithm tried to determine that, it probably wouldn't really feel right - a game must be careful about its use of procedural generation. I've played many games where too many pieces are procedurally generated, and so nothing feels unique. As much as programmers try (at least with traditional, non-AI methods), we cannot beat the feel a human designer can achieve for items, enemies, places, etc. My point here: to reward exploration of interesting features, I need to manually design both the trigger of the reward and the reward itself.
One system for these rewards in EXO has two pieces, "Augments" and "Effects". Augments are equippable, with only a limited number allowed to be active at once. Effects happen automatically upon unlocking them, and hence an unlimited number can be activated. Finding or doing something unique may trigger an Effect or unlock an Augment.
These triggers can be any number of things, like surviving a fall on a planet with very significant gravity or discovering an object with a complicated orbit. Variation in generation should mean that when you're starting out, you might unlock quite different Effects than your friends (or than your previous playthrough). Exploration is random and I want luck to play a significant role in the player experience.
However, I quickly realized that having only the random progression through Augments and Effects felt too arbitrary. I had basic capabilities, like the strength of your scanner and the temperature range of your spacesuit tied to Augments. But this way, if you, by chance, find a highly unusual planet in your first system and unlock a powerful Augment, it might basically skip you forward - and some Augments that could have been exciting will not be. I realized that I wanted the, um, effect of my Effects and Augments to provide a more lateral and branching progression, where the player receives some feature or ability that they can probably use in certain situations, but it's not a straight increase in power.
With that, though, some sense of progression was lost. I didn't feel like I could work toward anything, since it was more or less random whether I would find something that would unlock an Augment. I needed to augment my Augments (heh, that one was on purpose) with a more traditional linear progression system - money, items, and equipment.
Exploration still needed to be the means of progression, though. Rewarding the player with items for exploration is not too difficult - "chests" (in this case, lost ship cargo and the like) have been used in games forever. But, item-hunting is not the spirit I want for this; the "hope I get lucky" feel should be for "hope I see a cool thing". But the game should make you want to see a cool thing because it will give you a reward you want - and if it doesn't happen to unlock an Augment or Effect, I don't want there to be too much disappointment.
Eventually I landed on a borrowed feature from Elite Dangerous, where just by going to a place and scanning it you get "Exploration Data", which can be sold. So money (perlite) became a thing. This way, rarer features can be worth more, and every time the player finds one they get a reward. They also have to find an alien race that will pay for the information, but that is relatively straightforward. Of course, money that can't buy anything isn't money! So I placed the main, more or less linear progression in items that could be purchased. These can be straight power increases, unlike Augments and Effects typically, and they control all sorts of basic capability, including the scan strength and temperature control mentioned previously, as well as your ship's jump distance, cargo size, your time on non-breathable planets, and more. Some items still contain tradeoffs - which if you read the post last week, you'll know are important - but items of a given type are organized more in "tiers" of power, with a few tradeoff variations in each tier.
It's nontrivial to actually design all of these items that make up the linear progression, but I won't delve into the detail of those decisions here. I'm constantly considering the ways to feel progression since I have loved RPGs for those little level-up dopamine hits for years - so you can be sure that EXO's progression system will be at the very least well-thought out.