Obviously there are loads of things these classes are supposed to be able to do if they existed in the real world but only recently have fire spells been given the ability to light things on fire if flammables are on the ground. I’d say to give it time but another obvious thing is that there is no way all of the possible things will be added.
Kelvinists should be able to cook and freeze food, as you’ve said, they should also be able to freeze bodies of water so you can walk across them and create steam explosions from water on the ground to provide cover in a similar manner as smoke bombs but also as a possible actual damaging explosive. They should also be capable of boiling an enemy’s blood/blob until they explode or die in a similar manner to a steam explosion. Two abilities they should most definitely be capable of is making their body colder or warmer as the situation demands until they’re comfortable. Lava shouldn’t be a issue to them either, simply freeze it. They could also turn the floor into lava by rapidly heating mineral ground to the point of melting. Need walls? Ice! Need a home? Igloo! Need to forge? Heat and bend the metal with your hands, dimwit!
And that’s just kelvinists. Of course there’s loads more but I’m not going to write a book on kelvinistic theory. Imagine what would need to be implemented to allow all that, and then think of the other classes. I’m all for these nifty things but I don’t think these can be added via modding and will require some infrastructure in the code to allow that sort of thing.
You never know what the future holds, but I wouldn’t hold my breath unless a dev found these cool enough to be added.