That's a real super powered Feat, interesting concept but would seem to fit the Order of Elementalists or Elementalist School as Features. I also don't think the barred opposite elemental works. A Fire Mage with advantage on their Firebolt cantrip or Fireball spell works, I would suggest that they receive disadvantage on defending against spells of the opposite element. This way the onus is on the DM to challenge the Fire Mage who may even have a rivalry with a NPC Frost Mage or Frost Sorceress.
For that 1d4 boost, I could suggest that there are limits with it. You roll at the beginning of your day and get a boost of 1d4, which is your total boost for that day.
You roll a 4, then you could spend it in one go and cast Thunderwave as a 5th level spell or you could split that 4, maybe do Thunderwave at 4 and then 2. The limit I would propose us that it cannot be used to boost a spell beyond 5th level or be used on 5th level spells. You may or may not like this, but it's an interesting option and seems to fit in with 5th level being the sweet spot of magic now in D&D, it also means that the Elementalist has a degree of control over their power.
An alternate idea could be to check out the possibility of giving the Elementalist a small amount of Sorcery, giving them some limited power points and very limited meta magic. This could be an interesting experiment.