2018-01-19 : 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.

2018-01-19 : 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.

2018-01-19 : If you make an invalid choice, you can still make a valid choice as long as time hasn’t expired.

2018-01-19 : 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.