I know several have suggested this before, but I wanted to organize the ideas here and hopefully make it easier to understand the request. Please reply with additional ideas or corrections. Also, I don't know how much work it would be to implement these ideas. If it's a few minutes per card, then it might be worth Andy's time. If it is much more than that, then maybe only the most useful ones should be displayed.
I've broken the towers into groups with a decreasing priority level.
Cards without "custom bonus" that should have a useful "custom bonus"
Beaver - current stun chance
Poison Frog - Poison damage % (similar to how solara has burn damage %)
Herb Witch - Current speed bonus
Elvis - current slow time
Irish Pub - current damage bonus
Money Bin - current gold bonus
Scarface - average multi-attack. The average could be calculated from luck.
Twisted Novice Wizard - Current chance for a good / bad spell.
Small Spider - Current slow %
Adventurer - current bonus item chance?
Gargoyle - Current knockback chance / distance
Guard - Guards in range or +base damage bonus
Tinker - Current luck bonus
Templar - same as guard
Phoenix - Rebirth count
Candle - Flying creep detour length. - if that isn't easy to show, then flying % damage bonus.
King Arthur - same as guard
Cards without "custom bonus" that could have one for fun or tracking a statistic
Beaver - Total stun time
Scientist - xp earned
Pickpocket - gold stolen
Scarface - highest multi-attack
Black Widow - Number of seduction kills or number of bonus items found - if that is possible to determine.
Miss Jilly - Number of creeps killed with DBT.
Twisted Novice Wizard - total spells cast good / bad.
Dark Forge - number of items forged
Adventurer - number of bonus items found? - if there's a way to track them.
Rainbow Unicorn - Creeps impaled or rounds survived
Cards without "custom bonus" but that shouldn't have one.
Dandelion - n/a
Ganesha - n/a
Manitou - n/a
Yggdrasil - n/a
Hitman - n/a
Blofeld - n/a
Acolyte of greed - goblins spawned?
Dark Lucifer - n/a - unless chain number is allowed to grow, then it could be current chain attacks
Training Hologram - Total shared xp?
Snow Globe - n/a
Lucifer - n/a
For reference, below are the cards with "custom bonus"
Baby Rabbit - number of carrots
Wolf - role in pack
Huli - Bananas eaten
Bear Hunter - number of traps set
Holgar the Horrible - Number of mead consumed
Balu the Bear - Cuddles
Electric Chair - Chain attacks
Muli - liquor consumed
Mr. Iron - construction time
Stonecutter temple - Members (maybe total member level would be nice instead?)
Scarecrow - number of crows
Shadow - Adapt %
Gib, the frozen daemon - max stacks (I think?)
Solara - burn damage%
The Ripper - thirsts
Knusperhexe - mass creeps eaten
Abyss King - Army size