I'll cut straight to the chase:
I'm red-green colorblind and I can't see the red "attack incoming" indicators at all.

If I'm in duel mode with a friend or a bot and I take it really slowly, I can spot them. In the heat of the moment, though, especially in big brawls with a bunch of stuff going on, the indicators are totally invisible to me. I'd honestly be fine if they stayed white and just got bigger. That red is one of the exact reds that my brain does not like.

Also, in bigger skirmishes, the icon that shows a player's team needs to be much more pronounced, as that's essentially the only way I can tell if that dude running at me is a teammate or not. (Aside from the rare circumstance where my team and their team are completely different heroes.) I would love it if healthbars were color-coded by team as that big stick is far easier to spot than a little icon.

Finally, the guard-break indicator is actually so tough to see for me that I didn't know it existed until my friend made an off-hand comment about it. Would be great if it was a different color, had a border, glowed brightly, I'll take anything.