Dr Beaker already has one… I guess I can make Klaus wear it, too.
If by “witch clothes” you mean “recolour the shirt from white to black”, then yes, this is already done. And the flasks now contain blood*
* This, together with other Halloween changes, are problematic because they potentially raise the game’s age certification. It’s not an issue this year, but I could get into trouble with one of the app stores next year, so enjoy it while you can.
It already is.
That’s probably too much work.
As much as I’d like to implement everyone’s suggestions, the truth is that Halloween is not a major holiday and the time I have available for the re-skin is very limited (new update is coming probably on Friday). I’ve already broken the rule of “no code changes, only texture changes” for this edition.
Fixed in v.121 
Bug
Uh-oh… Chillers is the correct behaviour… Toxics are wrong, though. It’s complicated to explain why, but the end result is that if you kill a Toxic with an ICBM (direct hit, not AoE), then you can see the toxic cloud, but it is in fact harmless. Try it. Fixed in v.121 Bug
Fixed in v.121 Bug
This is the “lost profiles” bug back from Early Access version 36 . This has never been fully fixed (just improved because I now keep a backup copy – but in rare bases both the original and the backup are bad)
Should have chosen then cloud. Files stored on the cloud are not susceptible to the “lost profiles” bug.
You mean accidental after clicking twice to confirm? 
But seriously, it’s a good idea but not worth the effort (it’s vulnerable to cheating, and it can’t be easily secured).