For Doom, I'd imagine its powers would revolve around decay. It could be rotting things, crumbling them to dust, or maybe even directly aging foes. On a more general sense, it could also just allow the character to weaken foes with some kinda dark-green Doom energy.

Or, you know, bombs. Or coding. Those are both pretty big Doom themes as well. A god-tiered Doom player could be walking around hacking into reality and tossing bombs out of thin air!

For Rage, you could just do the inverse of Hope's silly nondescript white magic/science with spooky black magic, but honestly, that feels a bit too easy -- having showy white magic that doesn't really make sense but you believe in it anyways strikes me as more of a Hope-specific thing than a Hope/Rage pair thing.
Personally, I've always thought lightning or fire would fit the best, but I've read a few good arguments that sound is the medium of Rage powers. Certainly fits with the Makaras and their notrious honks, at least. So maybe Rage powers involve yelling and/or honking really loud!

Blood is a really tough one, honestly. It could just be actual blood. I remember seeing at least one person before say that Blood powers could involve chains, which I do like as well. It'd fit with the restrictions/bonds idea... a very literal and forceful way of creating bonds!