Meta-cardbonus!

[Phayte]

It could be cool if there was a meta-bonus for reaching a card milestone on every mob on a planet (including bosses). Maybe a percentage to planet-/total damage bonus, or a separate flat value, or 100% to the total bonus damage from that the specific tier (as in, get 10 cards from each mob on a planet with 10 zones, gain a bonus of 10x4. Get 25 on all, gain 10x16. Get 50, gain 10x36, etc).

It would be an incentive to farm bosses, though the extra bonus would have to be significant to make up for the horrible xp+gold/hp ratio on bosses.

And obviously there would have to be a meta-meta-bonus, where you get infinity damage on reaching 10 cards on every mob on every planet, with an additional +infinity for each following milestone. 

[WhiteK]

In the future it sound good but the + amount kinda high and i hope boss node mechanic will be change in the future too * like higher exp + gold card drop rate *

[Phayte]

With the bosses as they currently are (4x hp, 1x gold/xp), you're sacrificing a lot by farming them, so the reward would have to make it worth it.
You also would have to farm ALL the bosses on a planet before you'd get the reward.

Let's say you're doing Inferno 2, Planet 1, where the average number of cards per drop is 2, or 1 card per 250 kills. You'd have to kill each boss 2500 times to get the lowest tier of meta reward, or +40 damage. 12500 of each boss to get +160 damage, etc.

The bosses on a planet have roughly the same HP as all the other mobs on a planet combined (actually about 5% more), so it's like having to kill every regular mob on the planet all over again, except this time with a quarter of the reward in gold/xp. Getting a nice chunk of extra damage after all that sounds reasonable