My suggestion is some kind of "Crusader Coin" you can buy with real money to buy some upgrades or advantages.

$5 - 500 "crusader coins"
$10 - 1000 "crusader coins"
$20 - 2000 "crusader coins" + 2 jeweled chests
$40 - 4000 "crusader coins" + 4 jeweled chests
$80 - 8000 "crusader coins" + 8 jeweled chests + reset cool downs on all your skills.

$1 = 100 crusader coins and if you buy a large amount at once you get some extra stuff to encourage players to spend more.

Here are a few things I think these coins could be used for:
1) Spend 1K coins to reduce costs of levels by 10%. Can be purchased 5 times for 50% price decrease. This will be permanent.
2) Spend 500 coins to permanently reduce time needed for XP points by 10%. Can be purchased 5 times for 50% reduction.
3) Spend 1K to permanently increase chance of epic cards from jewel chests and rare cards from silver chests by 1%. Can be purchased 10 times for 10% increase.
4) Spend 2.5K coins to boost monster spawn speed by 25%. Permanent. Can be purchased 4 times for 100% increase to speed.

Feel free to add more to this short list.

What are your thoughts? do you like the idea of coins but you don't like my list I have suggested? please share!

I am a big fan of P2W.

dunno about the coins but i would definitely pay to increase the base speed of the game... the premise of most idle games is wait or pay, and i am starting to get bored waiting to accumulate levels for idols :)