Here is the card as it appeared in Stronghold design: Mind of the Controller WUBRG Summon Legend Gold 7/7 Trample Counts as a Sliver 3: Put a Sliver token into play
Just like persist on Putrid Goblin, undying is easier to break than a porcelain vase in a Matrix movie
That makes playing aggressively against your rival players far less costly, but can also be used tactically to deliberately take characters off the table to reactivate their effects as they enter or leave play - as with Merlin, Goat above
Legendary Artifact The Ozolith Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith
There are a ton of cards that care about enchantments in both green and white, and cards like Yavimaya Enchantress and Ancestral Mask help to promote building a huge creature that would love to have lifelink and trample
There are many niche merfolk support cards that will basically only see the light of day for a product like this, so Wizards of the Coast had no reason to leave any of them in the vault unless they break the blue/green color identity (sorry Sygg)