TL;DR: As a DM, how can I prevent a full single-element caster from becoming almost useless against foes immune to his element of choice, without making him overpowered? To make the question more general, I wrote "caster", but in this case it's a Sorcerer.
I'm DMing a newly-started D&D 5E campaign with 5 new-to-the-game players, one of which ("Mark") chose to be a Sorcerer. Mark has a particular love for electricity in general, so he thought of a character obsessed with lightning to the point where he refuses to study (therefore to know) any non-lightning spell, thus all his attack spells will be limited to those dealing lighting damage (e.g., Lightning Bolt, Shocking Grasp). He doesn't want to change other damage types into lightning (i.e., no lightning Fireballs), he simply won't learn any attack spell that isn't already based on lightning and doesn't deal lightning damage. When there's no choice, he will opt for utility spells. Even further, he will never do any attack that deals any other damage type except lightning.
Now, that's great, I think there's good roleplaying potential for such a character. However, as a DM, I'm concerned about his combat usefulness against foes immune to lightning; I guess this comes from my inexperience. Yes, he could have a few utility spells, but I don't think they make up for the complete loss of damage. I warned Mark that enemies with resistance to lightning might show up, prompting him to choose Elemental Adept (Lightning) as a feat, which he understandably found fitting for the character anyway. However, I added that enemies immune to lightning might show up as well and Elemental Adept won't be of any help in such cases.
Mark said he doesn't want to give up the lightning-only trait of his
character and would like to be helped figuring out something.
He is level 2, as the whole team is. I haven't considered magic items
I want to find a way for his character to still be useful against lightning-immune enemies (e.g., the Kraken, which I plan to unleash further down the story), without breaking him in terms of power. I thought of different options, for example:
Elemental Adept ignores simple resistance and reduces immunity to resistance; at the cost of 1 Sorcery Point, he can choose to turn the damage type of lightning spells into force, but as this is my first time dealing with such a case, I'd like to hear some opinions and advices.
If you don't use immune foes when you were planning on doing so then you are invalidating this players choice, and taking away the agency they showed during character creation.
You will also impact other players who might wonder why you aren't skirting around their weaknesses - or worse, why they bothered to create rounded characters instead of finding a niche and expecting you to cater to it.
Both the player, and the character will know that there are consequences to their choice of element, so just let it be.
This way you give the player the option to pick the Elemental Adept feat (Assuming you allow them) and double down on their niche. The player is thus empowered to overcome the weakness, rather than you simply ignoring it.