The Mr Freeze saga ended with ''SubZero''
As far as I'm concerned, this is where Freeze's storyline stopped.
What the team of writers did with the future series (ie. The New Adventures of Batman, and Batman Beyond) was absolutely ridiculous.
They effectively discarded everything that Mr Freeze was about, like they ignored everything that happened in the original series.
Writers: here's a bit of advice - know when to leave a character's story alone.
If you really had to revive the Mr Freeze character, you could have done it with another character (ie. a protégé who undergoes the same transformation as the original Mr Freeze).