Wavecrash titan?

[Wingnut]

So say I trigger {wavecrash titan}'s heroic, can you choose a creature that is already tapped to make it so that it doesn't untap during their next untap step?

[Keyeto]

So say I trigger {wavecrash titan}'s heroic, can you choose a creature that is already tapped to make it so that it doesn't untap during their next untap step?

Sure! If it doesn't specify "tap an untapped creature" then you're good to go.