I am creating a fire Genasi Evocation Wizard. I'd like to customize it a bit - and handicap myself - by only learning Evocation fire spells (no lightning, ice, etc). This is obviously fairly limiting and also screws me against anything with fire damage resistance/immunity. What would be a fair way to balance this out? I was thinking something like Elemental Adept and Portent. Too little? Too much? This is the first time I'm playing around with customizing a character like this so any help is much appreciated! 

Elemental adept works wonders when you are running a single element caster. If you are looking for something more, you can do what I had a fellow player do in a previous campaign and have a "secondary school" where they don't get the subclass abilities of that school, but you can pick spells from another school of magic to round out your spell selection. In the campaign I was in, the player was primarily an Illusionist, but only took damage spells that were fire. As for me, I run an evocation fire wizard too, but I have them also pick conjuration spell so that he can also make demon/elemental summons to help out.  When running a character like this, it's fine to have a focus, but there's no issue in having other spells with a different flavor, such as when you cast shield instead of an arcane barrier its a temporary molten armor.