I feel obliged to offer this helpful hint on the topic of "think"
vs. "thing".
The idiom is not merely "you've got another think coming." The idiom
is "If you think [something obviously wrong], you've got another think
coming." To bring it back to the fic at hand (and paraphrasing badly):
"If you think I'm going to let you and Gohan run towards the sun at
supraluminal speeds, you've got another think coming."
See? "Think" in the first bit matches the "think" in the second bit.
Arguments about mishearings, or proper-grammar-by-use don't really
count here. The idiom depends on _both_ "think"s.
(Note that the first think may not actually appear, as in "You've got
another think coming, buster!" but realize that it's a reference to
the full form as given above.)
Try it out:
"If you think Shabranigdo has your best interests at heart, you've got
another think coming."
This makes sense. Now replace the second "think" with "thing".
"If you think Shabranigdo has your best interests at heart, you've got
another thing coming."
You end up with something that only makes sense if the speaker is,
perhaps, a UPS agent. There are idioms that make no sense, but this
isn't one of them.