plain sinew
My favorite part about vanguard units Is literally just their custom ui color in your hotbar. I've seen suggestions like this before, but I want to spread the word.
What I think would be cool to be added for future vanguards or vanguards we already have:
Custom upgrade UI, Like sukuna's little line pattern at the bottom. Doesn't have to be special or anything, like a creeping shadow at the bottom for Jinwoo, or flames at the bottom for Escanor.
Even more custom hotbar UI! This one might be a stretch, but in the actual box where the unit is, instead of the normal glow that it shows, it could show something else. Example: if we got a lightning vanguard unit instead of the glow effect around the box it shows little sparks of electricity. (very subtle)
Custom ability/bar ui: little changes like an outline, shape change, or color change to visuals like a gauge or an ability. Example: Jinwoo's mana bar being the same color as his hotbar outline custom color, or Escanor having his abilities being engulfed in flames or something.
I've seen this everywhere, but custom trait colors for the vanguard unit, like Kaguya's white and pink/purple being applied to the monarch trait in the hotbar.
Again, I've seen this in a different suggestion, but for units with vanguard familiars equipped, change their outline to be the same vanguard outline as the familiar. (Like gojo's outline would become blue with the glasses equipped)
slate marsh
swift python
inland hill