No matter how much you increase the rewards, any good-reward game mode or fun game mode will be dominated by the mass p2p. The only real way to tighten the gap between f2p and p2p is to have the p2p players boycott for the f2p, but that won't happen so no luck there.
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But if you want some better rewards, maybe these are some ideas:
Quest Board - as of now, you get about 7 100k books and 35 20k books for 500k might. Obviously someone of that stature doesn't need that. A constant struggle I find for f2p is leveling heroes. Granted this only takes time, rather than just being impossible, it is not a widely-complained-about issue, the Quest Board should have much better Experience Book rewards.
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Sign-In Rewards - change all of the green fame boxes from Sign-In rewards to Blue fame boxes. That is about 7k fame per week, giving players more incentive to sign in everyday.
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Dungeons - All Shard amount multiplied by 1.5x and Honor Badge amounts multiplied by 10x. Don't think that's much to ask.
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Better items in the Merits shop. They are still selling Level 1 crests in there, and 100 Honor Badge Packages. Don't think anyone is interested. (though the semi-frequent refresh cards and such are nice sometimes)
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Lost Realm - multiply Red Crystal winnings from battling teams by 3x, blue crystals from farms and altars by 3x, and add stronger teams that can give some new rewards - such as 5x Ignite stones or 1x Lvl 8 rune (make these teams more rare, of course). Those Double Evolved teams aren't impressive.
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Forgotten Trials - for the amount of hours you have to put into each level, a few hundred fame and gems doesn't really cut it as an incentive. The only incentive to getting higher is as status symbol. Give a more exponential increase in fame/gems/birthrocks. Perhaps a base line could be FT Lv20 gives 2000 fame, 400 gems, and 80 rocks. (if something like this is implemented, simply give extra rewards to accounts that have already beaten past levels)
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Castle Crisis - First of all, unrelated to rewards, I believe the first 3 stages should last a flat 5 days and the Demon last 2 days. Anyways, Ember Army - Merits are still not viewed extremely valuable even after the wardens have been implemented, as you can buy enough to max them all for $5. I think a good reward for Ember Army would be fame, rather than from Archdemon. As for Avarcifier, just up the levels or make them Insignias rather than crests. Impster rewards aren't shabby - chests can be pretty chinsey on good rewards though. Archdemon could have Ignite/Zenith stones as a reward, Rank 1-10 getting around 500 Ignite and 50 Zenith, rank 11-50 getting 300, 30, Rank 51-100 get 200, 20. So on.
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Arena - Arena is one of the most competitive game modes (especially at top-tier) but gives horrible rewards. 80k hb and 200 fame per week for the top ranks is nothing short of unimpressive. They recently did "update" Arena to have the top 3 displayed which made it even more competitive, but still bad rewards. Possibly give 3000 fame for rank 1-20 as a base line and 20k shards.
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Guild Wars - One of the more complained-about game modes, I think guild wars should cap at 4-5k fame and 50k honor badges.
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Arid Ruins - a semi-recent game mode, some were excited for a change of pace with a new mode, but the rewards weren't good enough and you can beat stage 1 and Leave for about 70% of the rewards which is what most do. Should be more incentive to stay to wave 5. Wave 5 (and other waves) could become harder along with better rewards such as fame and larger quantities of soulstones for better heroes.
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Just some ideas. Maybe some good, maybe some bad. Give feedback if you'd like
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Maybe not the greatest idea, but Bazaar coupons could be a thing. Where every month or 2 months or on Holidays, IGG gives everyone a $5 or $10 coupon they can use on a Bazaar. I think a good system for this would be as a usable item that can only be used for that day's Bazaar and will expire after the server resets. When used, it gives the rewards of a respective 5$ or 10$ Bazaar from that day. Example: if a $5 Bazaar gives 200 blue fame boxes, 200 pet bag IV, 2 Prime hero card III, and 100 Gold books, the coupon could be used and those rewards would be given. Also I think these should be exclusive to the coupons, rather than being able to buy it.