To the Translator and/or Proofreader
You were on the right track but-
design
1
: a particular purpose or intention held in view by an individual or group
"He has ambitious designs for his son."
Webster says this use of the word is correct, but there is a grammar rule involved. To be fair it may not be stylistically popular but there is nothing technically wrong with it. The problem is, having a design ON someone would usually imply something involving clothes. Having designs FOR someone isn't wrong, even though you'd normally use the word "plan" over "design". Regardless, the word "ON" was used instead of "FOR".
My guess is that putting "I've had plans on what to do to you" would have been too long-winded for the speech bubbles. But that's too literal of a translation to begin with, and in English we have a turn of phrase or two that equates such as "I've got plans for you".
I'm sure most people won't have an issue with it. Sadly for me it was awkward enough to distract me just as it was getting good. ^^"