Warning: Wall of text incoming (theres a one paragraph tl;dr at the end).
Heres the idea: What about some system where you would have like an ongoing challenge to complete while playing the game. Once you complete a task you get a reward and new random task to complete. Unlike guild task they would however not increase in difficulty. Instead they would be split into three groups:
Easy Task
It would be something simple you dont really have to think about. It would complete itself eventually while you play and give symbolic rewards. It could be something like:
Make 25 green matches
Match skulls to attack 15 times
Win 5 battles
Make 4-match 20 times
Inflict 200 damage
Essentially something you dont have to focus on too much, since it will complete “by the way” eventually. The rewards for this easy task would be something small, like between 50 and 100 gold, or 5-10 souls, or minor traitstone.
Medium Task
Medium tasks would essentially be easy tasks on steroids. They would be similar in nature but would require longer period of time to complete. If you dont focus on them, they would complete eventually as well, but it would take much more games to do so. So while easy tasks can be completed in about 3-5 games, medium tasks would take maybe something like 15-25 games. And if you focus on them a little bit, you could complete them slightly faster. These could be:
Match skulls 100 times.
Use troop ability 150 times.
Gather 500 blue mana.
Win 20 battles.
The rewards for these would be much better, but nothing extraordinarily generous. It could be something like: 2-3 gold keys, random rare troop, 50 souls, one glory key, 30 glory, 2-3 gems, random runic traitstone, a treasure map or similar.
Hard Task
These tasks would be extremely long. In fact they would long enough so that it would be worth to create a team specially composed to complete the task. First of all it will mean that people will start to experiment with more team compositions, and also all those stupid team slots will be suddenly useful, since you would use to them to store teams for specific hard tasks. The requirements would be something like:
Make 1000 purple matches.
Inflict over 9000 damage. (pun intended)
Explode 500 gems.
Use troop ability 1000 times.
And so on. Unlike easy and medium task, some of those would not be completable just by the way (like the explode gems task, or there could be task to destroy or create or change colors, or task to steal attack or buff own attack). Those tasks would take so long that it would be good to create special team as mentioned. Which means that the rewards should also be worth the effort you put into completing them. On average these things would take like 100 games to complete when focusing on them and even more if youre just trying to complete them “by the way”. So I think the reward could be something like: random arcane traitstone, event key, gem key, 250 souls, 10000 gold, 100 glory… and the most fabled prize of all: 5 VIP points.
Conclusion
So… at any given time, you would have one of each - one easy task, one medium, and one hard task. Easy tasks would complete fast and would give crappy rewards (similar to rewards ingame for completing battles). Medium tasks would take much longer and would give decent rewards. It would be decent value, but compared to tributes it still wont be anything super amazing. Lastly the hard tasks would take so long to motivate people to build a team designed to complete the task faster and would give very good rewards (considering how many hours are necessary to complete them).
PS: I know giving 5 VIP points for the hard task might seem weird. But consider that only like 1 out of 10 hard tasks would give you the VIP and each one would take like 5 hours to complete. That means you would get 1$ value from playing the game for 50 hours, which is really nothing in my opinion. Any sane person would rather spend an extra hour in his regular job and then get 10$ worth of merchandise and VIP points instead. I mean thats 500 times more efficient than actually trying to farm hard tasks to get VIP… the idea is not to give you “fair” value for time sunk into the game, but to reward you with something worthy as an appreciation of your dedication.
TL;DR
Three tasks (quests) at a given time. One easy, one medium, one hard. Easy task gives crap rewards, medium gives decent rewards but nothing too amazing (similar to tributes), hard tasks takes super long to complete so its worth to create a special team for it. Gives good rewards (better than tributes or arena).