Hi!

I'm working on a homebrew subclass, and I was wondering if there's a way to give the subclass an immunity to extreme cold and extreme heat, apart from giving them resistance to cold and fire damage.  Any idea where I might be able to find that?  If it's just something that has to be written in the flavor text, that's fine, but if I can have it listed under the resistances box, so much the better.  

 There's this suggested possible minor property for created magic items:

Temperate. The bearer suffers no harm in temperatures as cold as −20 degrees Fahrenheit or as warm as 120 degrees Fahrenheit.

You could use the "suffers no harm..." part in the description of the subclass feature. Note that this won't count as resistance to either fire or cold, but it will be in keeping with how that kind of temperature-tolerance benefit has been described in the rules.

Edit: As an example, our party has an awakened shrub with a homebrew Temperate Kilt, for dealing with those extremes :D

It would just be flavor text, only actual damage resistances actually appear in the resistance box. For example, before they changed things, the Goliath used to have this instead of resistance to cold damage: 

You’re also naturally insulated against cold climates, as described in chapter 5 of the Dungeon Master’s Guide.