For base, I'd suggest a factor for a combo base - - angelic or demonic are the top options right now since the aether/nether combo bases are very new, but I've noticed a new combo base gets released every development update, so you could either go for one of the ones that exists now or wait a bit and see if there's something newer by the time you're ready to king that guy!

For eyes, I am a big fan of "arid" - - but it's very expensive... combo eyes exist, too, and a factor for one of those might make your heir more valuable (this includes arid, it's a factor for one of them but I forgot which one)

For mane, I think the shape you have now is great - - you can get the mane succubus lioness mutation with it, and that is my personal favorite mane mutation ! You could give him a different color, probably one of the event applicators, but this is more of a bonus than anything. Chocolate is a common mane color, though, and changing it would probably increase your heir's value

For markings, getting the full 10 is the way to go - - anything beyond that would just be bonus (and would look pretty neat if someone is looking for a stud and stumbles upon that guy). Tier 2 and higher is what you want, but anything you can get an applicator for isn't as valuable (like the ones sold in event shops). If you really want to go hard, you can use random marking applicators and try to get some breed only marks or rosettes! Those two are the most valuable (rosettes in particular), and having the full 10 is really good - - any really special markings you should set at a higher opacity, too... don't entirely know for sure if this helps cubs inherit them, I am just under the impression that is true

For stats, do the cub training every day until he reaches 1 year, then send him on patrols with your submale for experience - - make sure he's over 75% of the cub training bar by the time he's 1 year 11 months so he can get a little stat boost aging up ! You can buy medlar fruit in the manticore shop right now to help you out, or you can get it off branches. I got this boost with two of my lions and I would say it's worth it ! Make sure to patrol him all the time until you retire your king ! I'm not a stat guy so I don't have much to help with on that ^^' Appearance is the most important to me and stats are a really neat bonus, but that's just me

I didn't add any new markings, just set them all to 100% (I do not know what markings to add specifically), and threw one of the backgrounds on the top on him... he also has new base, eyes, and mane color while keeping his vibe ! The image is clickable if you want to see what I used