If I hold with Rumble, Hothead to trigger Reckoner's Arena, which activates Hothead's ability to play Rumble, Scrapper from trash, can I then activate Rumble, Scrapper's hold ability?

Ruling: No, you cannot activate Rumble, Scrapper's hold ability because you have already held this turn to trigger Reckoner's Arena.

Sequence:

• You hold with Rumble, Hothead
• Reckoner's Arena triggers and goes on the chain
• When Reckoner's Arena resolves, it finds all conquer effects of units currently in play and puts their abilities on as pending
• Rumble, Scrapper played from trash after this point misses the Reckoner's Arena activation window

Nuances:

• Reckoner's Arena cannot "see into the future" to know what will resolve after it resolves
• The timing for a hold trigger is already past once Arena has been activated