Good idea for a thread.
Perhaps some more organization to prepare for an overload of ideas to review?
Make a spoiler-ed list of the ideas for review in the OP. Have 1-3 ideas linked above each page (use a poll with no votes) to be discussed.
Have some means to identify when a card is done with the reviewing.
It was/is a good example of two half effects in one card. (Horned Frog + Heal -> Forest Dryad)
The upgraded's Deja Vu ability probably distracted from the core idea of the card.
Also balance was an issue because they were both UP by 1|1.5 IMO.
3 Horned Frogs + 1 Heal = 8

+4 cards +20 healing +12 attack >> 4 Forest Dryads = 12

+4 cards +20 healing +8 attack
[net difference 1

+ 1 attack per card which is too high by 1 IMO]
1 Giant Frog + 1 Improved Heal = 4

+2 cards +20 healing +5 attack >> 2 Forest Dryads = 8

+2 cards +20 healing +4 attack
[net difference 2

+ .5 attack per card which is too high a difference.]
Also 5 healing is not that valuable compared to 20 healing. I think 10 healing per Dryad would have been a better start point.
I would probably design the next version on the principle that 1 Horned Frog|Giant Frog + 1 Heal > 2 Forest Dryads.
However you were right that it would fit best on a cheap creature around the same cost as Heal.
Cost: 3

|3

Attack: 2|4
Effect: You Heal 10hp when it comes into play.
HP: 2|3
Net difference (from the new card's point of view): +.5

cost +.5 attack | +2

cost +1.5 attack