With Doctor Who, cascade represented all the timey-wimey time travel we know and love on cards like Into the Time Vortex
They're not unknown to make some good use of +1/+1 counters, which is where Kalonian Hydra gets into the fun zone
The Hat Lillie SR from Collection Moon (2016) is second at $7,385 PSA 10, followed by Acerola SR ($4,210), Rayquaza VMAX HR ($4,187), and the Moonbreon ($4,088)
However, I agree that her activation cost is probably too costly, and I haven't built the deck with enough 5+ power creatures to make her hit consistently
A common misconception is that a card is playable as whatever the visible name says it is
That means if you wanted to you could build a deck with fewer colors, then throw in a couple of your favorite off-color creatures for Ashling to cast