Modular Monstrosity
Oracle Text
Whenever an opponent casts a spell, you have five seconds to choose a keyword you haven't chosen for a card named Modular Monstrosity today that's been printed on a creature card. If you do, Modular Monstrosity gains that ability. Otherwise, Modular Monstrosity loses all keyword abilities.
Card Rulings
1/19/2018 You may choose any keyword ability that appears on any printed Magic creature card. You can’t pick keyword actions like explore or scry. If the keyword has a modifier, such as a mana cost or a number, you must choose a modifier that’s been printed. Sorry, “annihilator 500” fans.
1/19/2018 Each keyword can be chosen only once on any given day. And that counts every Modular Monstrosity you play with that day. You also can’t reuse a keyword with a new modifier. For example, if you choose protection from Demons and from Dragons (totally counts), you can’t choose protection from red later that day. Protection has already been used.
1/19/2018 If you make an invalid choice, you can still make a valid choice as long as time hasn’t expired.
1/19/2018 If you haven’t made a valid choice after five seconds, Modular Monstrosity loses all keyword abilities, including ones its picked up in other ways.
I don't look forward to repeatedly telling people this at a limited event until I start getting called the "no-fun" guy. Breaking this is trivial if you are allowed to pick different versions of the same ability (islandwalk! plainswalk! legendary landwalk! bands with other dinosaurs!; you'll never actually fail to name something). Without that challenge the card is too easy, and thus unfun for your opponent.
The concept is fun, though.