See Blinding Realization.

Just looking at your Diurnal cards, there's no unifying theme among them as to what mechanics or playstyles are associated with Day or Night. It seems like you want Diurnal to be based off of what you need at that moment, rather than what playstyle the deck is going for. For example, Airbourne Raidleader can be aggressive in both modes, while other cards in White lean towards aggression during the day. The same goes for Taj-Crop Sharpclaw - there's no clear "this is more aggressive" mode, or "you want this card to be in daytime" mode. You might consider that an upside, but your spell affects half of my creatures in varying ways, it adds a whole lot to the board complexity while not really adding much in the way of gameplay.

Secondly, decks will be highly encouraged to run day/night changing cards, even if they have little Diurnal cards to take advantage of that, simply because of how it will affect the opponent's gameplay. I don't want to feel like I'm putting a Diurnal card in my deck solely because my opponent is choosing day/night uncontested.

In other words, without a clear mechanical theme binding day and night, there's very little that Diurnal really adds to deck construction or deckbuilding, other than "you get to choose your mode, but your opponent also can change it back too". It's not something you can plan for during deckbuilding or drafting unless you're specifically drafting a Daytime/Nighttime deck. I'd like to see more emphasis placed on Daytime/Nighttime strategies.

Maybe creatures are stronger during the day? are stronger during the night? Nighttime creatures are a little more sneaky/tricky? Something that makes me want to draft cards that are better or work towards a certain goal in the daytime or nighttime rather than work middling-ly in both.

I agree with that. Diurnal design space is kind of limited because diurnal cards eat up design space twice as quickly. They're also kind of hard to balance on paper because modal cards tend to be inherently stronger. At the same time, it feels bad to just overcost diurnal cards because at any given time, they're only one side of a card, so they always feel overcosted. Diurnal was intended to be more aggressive during the day and more defensive during the night.

Night side should probably just be an 0/4 vanilla