So here are my suggestions, some are based in speculation but they still serve to give Valyrians some love:
"The people of Valyria were very strong in magic, and they would use their powerful wizards and their dragons in conjunction with their armies to conquer most of the eastern continent. Dragons were controlled by whips, magic horns and sorcery." - taken from AWoIaF wiki.
"Maester Pol's treatise on Qohorik metalworking, written during several years of residence in the Free City, reveals just how jealously the secrets are guarded: He was thrice publicly whipped and cast out from the city for making too many inquiries. The final time, his hand was also removed following the allegation that he stole a Valyrian steel blade. According to Pol, the true reason for his final exile was his discovery of blood sacrifices - including the killing of slaves as young as infants - which the Qohorik smiths used in their efforts to produce a steel to equal that of the Freehold" - On Qohor, A Wold of Ice and Fire.
- Glass candles: As long as there are living dragons roaming around the world, a practitioner of the Valyrian religion with high learning can look into the glass candles as a way of keeping him/herself safe from plots. Maybe there could be a building related to the candles themselves that the player has to build in order to have access to them, like the Observatory from the Scholar WoL, but with an actual use for it, like the Alchemist's Guild.
-Critical Success: Your grasp on the unknown is unparalleled and you see glimpses of your own future greatness. Chance to get Ambitious trait, some prestige and maybe a very small chance of getting a Valyrian Steel sword/Dragon Egg/Artifact, as in the candles directed you to where they were hidden.
-Success: Now you can predict all your enemies' moves, and this keeps you safe from plots OR battle in the same way Rhllor currently does. A chance to get the Zealous trait. (Everything is proceeding as I have foreseen).
-Failure: You see nothing, no matter how hard you try. This could give you either the Cynical trait ("Magic is truly gone from the world") or Stressed (as in the character spent a LONG amount of time trying to make it work, like the Maesters at the Citadel.)
-Critical Failure: You see vague, disturbing images of suffering and pain, you don't know if you can trust anyone anymore. Chance to get Lunatic and/or Paranoid.
-Blood magic and binding dragons; how about granting a very small chance that someone exploring the ruins of Valyria gets an artifact similar to Euron's? Seeing the system of storing "magic credits" was already implemented with Rhllor, it could also be used to see if the ritual would work:
You need to store prisoners, and once you think you are ready you can try performing the ritual to bind the horn to you. The need to store a certain amount of prisoners as sacrifices to even attempt the ritual would serve as a nice side-objective for players, and add to the expectation leading up to it. Or, to make it even more hideous and GRRMy, limit the sacrifices to children only.
-Success: The horn is... well, now bound to you. If you meet a dragon in battle and get the option to face him, a new alternative should appear letting a bodyguard/courtier/dynasty member blow it for you. This would basically be a very difficult to pull off "dragon steal mechanic".
-Failure: The ritual itself goes wrong, and you pay the ultimate price. (As magic in Planetos always resolves around a very high price, the penalty for failure could range from lunacy, disfigurement or even death).
And maybe this could be bound to a similar mechanic to hatching dragon eggs: if the player wants a higher chance of making it work there would be the need to travel the world in search of lore.
And if there is a way to let the game know who the horn is bound to, the player could even try to pull a Crow's Eye-Victarion with a vassal commander that happens to be battling a dragonlord.
The Mystic trait could be the requirement to even attempting those things, in the same way both Rhllor traits currently work.
And that's all I could think of for now.