Drawbacks that appear if you pay the life rather than the mana for a Phyrexian mana symbol could bring the mechanic into a more fun and stable design space for future sets
They can also be used in combination with the basic land types you already play
Being able to take out the smaller things and keep the bigger stuff about makes mass removal feel very one sided against the decks you have it in against
Sure, your opponent might have ten beasts in their green deck, but Callous Oppressor would still be able to steal their four goblins, three elves, and two other creatures that weren't beasts
Creatures that enter attacking are never declared as attackers, and as such, they won't cause Demonic Covenant's first ability to trigger
Abigale, Eloquent First-Year is a bird bard with flying, first strike and lifelink which - when she enters - puts those keywords onto another creature as counters and then strips it of all other abilities