- Smaller undertaking, shorter timeline - Missions that unlock tangible rewards could be cool. They could include burning mechanisms as well.
Ex. Collect these 5 Illuvials (could be thematic based on affinity or class or not thematic at all). Depending on the difficulty of the mission, they could unlock things like: % discount on master shard fuel or fragment requirements, or % discount on next travel to a zone (could be tied to the thematic elements, like air being % discount for brightland).
The Illuvials being turned in could be burned in exchange for the reward. Would essentially allow players to upcycle their assets while removing some of the asset supply. This could even be something as simple as burning, for example, 5 T1 Illuvials to receive a Cured Uncommon Shard. The same principle could apply to each subsequent tier, so that 4x T4's can be burnt to obtain Cured Resplendent Shard.
- Larger undertaking, longer timeline - Missions could also be on a "track", with a larger reward at the end, but requiring players to play through and complete some # of the requisite steps first. As an example from World of Warships - Sets of missions organized into campaigns. Each campaign has a relatively large reward for completing it. (Image 1) Within a campaign, there's a progression through 5-10 Missions. (Image 2)
Each mission has a number of tasks of varying difficulty, with a threshold required to be able to attempt the final task to complete the mission. Each task has very minor rewards associated with it, with the mission completion having a larger reward. (Image 3)
Doing this 1:1 might not be viable, but the important elements (IMO) are:
Small, achievable tasks, that work towards a larger goal, with a worthwhile reward for completing an entire set of objectives. The minor and medium rewards provide the carrot to continue working towards the large goal, in regular increments. In this way, players can feel like they are getting things done every time they have a session, even if it is a short session. Players who want to play for 12 or 14 hours a day can quickly achieve the larger objectives.
This last part might be controversial, but I think long-term milestone rewards could be related to Illuvials.
Complete a large goal, and you get a special S3 run with a guaranteed holo T5, or with a guaranteed dark holo, or with a guaranteed Illuvial with stats within 80% or 90% of perfect. Players would value those runs and likely be very diligent in preparation to run them.
Milestone-specific cosmetics would be really cool too, especially if they reflect the nature of what is being accomplished by the player. Ex. Collect all fire Illuvials, or all fire Illuvials as holo, unlock this fiery ranger skin, or this holo fiery ranger skin.