I'm a huge fan of the implementation of seasonal challenges. Benefits include:
1. Large EXP bonuses;
2. Bright dust;
3. Availability of all previous weeks, so anyone can catch up no matter when they join a season;
4. Clear direction on what to do if you want to grind artifact power; and
5. Challenges linked to every activity, so you can play what you want when you want and still complete challenges.
This kind of system is much, much better than a large volume of low-EXP bounties. It isn't the most creative solution to the bounty problem, but it is certainly simple and effective. Good job.
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Only thing is the challenges need to be truly seasonal. You can complete a challenge within in the season, but if they want to keep something relevant and lock the challenge to later in the season, you may be forced to complete challenges multiple times. I'd be fine with not being able to get the XP & bright dust until the week it is available, but it should be completed if done at any point in the season.