My idea for a T 10 doom would be to change it from a fiery horse to a Storm/Electric horse with the mane and tail a static lightning and have the body look like a storm cloud with lightning flowing through it.

you could make it the fastest horse In the game and give it a unique ability like instead of pressing F for instant Accel it could be a quick lightning bolt teleport a distance in front of you(within eye sight of course) but you could make it go horizontal or vertical. 
also the fore chop skill could send a thunderstorm in front of it that does damage kind of like an Hashashin and his tornadoes or just a single shock wave in an AoE Cone. Just an idea and let me know what you think! I think this would be an amazing horse to have and would easily be the route I go for my dream horse 

1)

Change the color of flame from red to BLUE (to indicate a hotter flame)

2)

Increase Doom damage by +8AP

3)

Give it a further fall damage threeshold (increase by 30%) so we can jump off of some mountains and high hills without fear of dying.

... and maybe:

4)

Make it look menacing & fearless, taking its clues after Sleipnir (Odin's horse)