My only suggestion is that I would suggest you let those who take Ritual Caster just be able to pick any mental ability stat to use for their ritual casting, rather than be forced to use the stat of the spell list they pick.
Does this actually matter? What rituals actually use a stat? I'm not saying there aren't any, but all the rituals I can think of just happen without a check or save.
Yep, but of course not just fire, but whatever damage type you select. Our group actually liked the idea of being able to penetrate the immunity of things like giants, elementals, and dragons. However, they would still be resistant to it-- so it helps if a caster doesn't have other options ready at the time, but hardly solves the immunity issue completely IMO.
The reason they specifically brought up fire as a damage type is that 5e spells are balanced around fire being the most resisted element in the game. So in general, fire spells are more powerful than spells of other damage types to account for this. Elemental Adept (Fire) is already core to certain sorcerer builds. It's tempting for a wizard (what with fire spells being so powerful), but it's a resistible temptation since wizards are often at their best when buffing or using battlefield control spells. Changing immunity to resistance would probably make this mandatory for evokers. Which means it's too good.
I'm not saying I don't like it. I do like it. I just think it's too obviously going to encourage a fire build. I think if you want to work around immunity I would probably make that its own feat with a prerequisite of Elemental Adept. Elemental Master or something. Maybe you even take reduced damage from the element at that point. I haven't thought it out, but you get the idea.