Weather events aren't really moddable as far as I've seen. There is a text file that supposedly determines how frequently events occur, but I have not noticed any changes whenever I mod it.

As for elephants freezing, if you could find a way to code that I'm sure the whole RTW modding community would want to have your babies. As far as I know, there is no possible way to make units take random damage depending on where they are or what time of year it is. I have tried many times to try and script earthquakes to happen whenever specific conditions are met and in a specific area, but I can't figure out the right code for it. Armies taking losses during winter and while on the march, IMHO, would be an excellent addition to any mod.