Well, I'd change all his markings to 100% opacity except Copal Quagga
The visual difference on your king wouldn't be much, but it would affect the cubs' markings, making them stronger aka more desirable

I would also change the mane to a special colour, like Bloodbourne or Demonic (I'm mentioning these because they are the easiest ones to get right now)
Again, not much difference on your king visually, but it would help to get "better" cubs

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I think the marking opacity is fine. Changing his opacity won't change his cubs opacity. When breeding, opacity is random from what I've been told and have learned. His mane is also special, and desirable! Maroon mane, from my experience, is a liked mane!
Teardrop is a pretty liked base, and like you said, buttermilk is perfect for breeding that. Your king already looks amazing to me. Sometimes stud requests can come and go though. From my experience, sometimes I will get a lot in one week, and sometimes I will get zero.
I don't think you should change anything about him, he has good stats, good marks, and looks amazing in my opinion! How much do you advertise him though? Advertisements, and how much you advertise, can change how many requests you get. You probably already know that, but advertise whenever you can without spamming chat with a stud ad. Also, if you don't have one, you can also make a stud post in the stud forum! Those posts can pull several stud requests as well, especially if you bump the post every so often!
I hope this was clear and a little helpful, if you have any questions about anything though, please PM me!

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I like him how you have him now. I do look at opacity, but looks likes yours are all 100% or mostly there.
I love nimravus markings, so if you had more of those it would be even better. I'll be sending a few your way now that I've seen him.
Love the maroon mane.

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