So I think most of the community at this point is settled on what it should look like as in a white horse with blue flames.  Thinking about skills for it though.  I think they should get a fiery cyclone skill that does an aoe knockback and dismount to surrounding players and monsters and then gives a small dot.  Put it on a 5 minute cooldown at least maybe as much as 30 minutes and give a little bit of damage boost because doom appears to be the PVP mount anyway.  Just a thought.

Imagine a cavalry squad 😂 just rotation of the aoe cc that would be epic 

I had a thought that Doom could go to being a lightning horse than maybe keep his stomp skill but with a lightning theme and as his T10 skill maybe a teleport after instant accel but thats just me I am a console player though and that would probably break my console lol but I like the cyclone ability listed above sounds cool.

T10 peggy has 3 unqiue skills and t9 peggy has two. With that logic t10 doom needs another skill and i'll first touch on his existing ones. I've heard people mention about giving it a fourth Instant acceleration after Dark Sprint. I feel like that would be a good idea to make it so you can extend Dark Sprint; It would be slightly faster than dark sprint and be constant while holding shift and cost stamina per second and reduce your ability to turn a by half. While using this ability you would be in super armor and your horse would deal physical damage and leave a damage over time affect in the area for a duration (fire trail). The horse it self would take 50% reduced damage while using this ability as well. It's other existing skill is Dark Flame Steps.; I think they could buff this skill to channel more of ur ap into damage as well as leave the spot burning for an extended period of time which would apply a debuff and damage over time affect if you walk through it. The last skill should be a unique one, maybe something like an empowered roar that demoralizes enemies stiffening them briefly and applying an accuracy/ap debuff and giving you an accuracy ap buff as well as keeping you warm in the new continent.

I can understand the water vs fire aspect but it really only fits dine because of its ability to create a path no matter where it goes (think desert, no other horses can go. ) However, dine creates this path of grass to walk upon despite terrain textures. Because of the water walking skill (additional terrain compared to t9 desert walking) she thus has the water theme. If the water mane was given to doom it wouldn't make sense unless they added water walking to doom instead..however I feel like this would contradict dine's purpose. She is technically a healer but yet the least wanted t9. The most of the time someone wants a dine for utility is because they can walk on terrain no other t9 can go and thats about it. Doom offers competition as pvp, 2 seater and being as fast as peggy. 
TLDR; Giving the water appearance to t10 doom would significantly reduce the will for t10 dine.

As for doom features, I'm trying to think of those "cocaine decisions" that aren't too powerful enough to make it feel like you have to get it but are cool to have (as the directors said in heidel ball)
Changing the animation to a teleport from a forth instant accell is definitely up there. Only thing is they'd have to make it the duration of an IS or a longish cooldown, otherwise it would feel like you had  to get it (compartively to other t9s speeds) 
As far as skin wise, what are the thoughts on a full on skeleton horse (with flames)? 
They can even make it look like it's decomposing, such as monster hunter's vaal hazak :

I'm not sure if many people would like that though hahah.

*Coughs* so Hello to all BDO players or whoever reads this, hope you'll find something interesting here .

Mythical doom,that magnificent creature should be the fastest horse ,as other two t10 have their special talents.The source of its hidden power would be the Sun itself ,just like a phoenix it would be able to ressurect ,rise from the ashes :) ,btw i like the suggestion with the drawing ,where the horse absorbs energy ,gallops all the way over the mountain and then when it meets the ground deals lots of damage,although it would be depending on the toppography a lot ,so making it as an independent skill would be good too .

Doom could use power of the moon and power of the sun ,to convert into powerful attacks or buffs ,with a cooldown ,the player riding a doom should have a faster heal/lower death penalty or dr for example ,so the horse would be a shield in some situations...resistant to some mobs regardless of the level.Long story short ...i see it as a mixture of dragon and horse ,changing colors while on standby and while in combat mode ,their legs and body would have scales ,and its manes and tail would be slightly longer ,animated lava cracks while sizzling hot aura wraps its whole body or black aura like Nightmare of Arduanatt .Personally i don't like a ghosty type of horse it would make it look cheap IMO ..Also,even though we already have a winged horse ,Doom might develop wings ,picking up from Phoenix mythology ,once she has finished absorbing the energy of sun she will rise ,could be a part of an explosive animation ,,its body would be covered in fiery feathers somewhere or even detailed dragon alike tough scales .Sher should not be a pvp only horse .Combat horse ? definitelly yes!

Idea: Riding a Mythical horse should have its own music and for Doom it should be E1M1

so now that the mythical dine is out :) time to decide new color sceme for the mythical doom eh? i think they should keep the black but change the flame color in all honest to a dark hue of blue for a nightmare horse or go out of the way and do purple or white. 

on top of color with the skill set to be added.. i think it should stay with 2 seater being as a dream its the only with a second seater so should stick to it with mythicals as well. with the extra 3 skills in mind.. could break the game add a 4th IA , maybe a 3rd leap to help get around turrain instead of going on road so u dont get stuck , add fall dmg reduction on the 3rd leap and maybe add an aura to it like a passive dmg dealer. ?