Not sure if mounts are coded in such a way that mounted combat is possible, but a Universal Tree for mounted combat might be interesting...
Of course, its probably a waste of dev time since it would only be usable in Wilderness Exploration areas. Unless they change the code to be usable in dungeon instances too!
Yes, most quests don't require mounted combat, but the few that take place in large outdoor area could be quite fun... plus, it would give someone a reason to equip a Shortbow or whatever melee weapon works best on horseback that is ignored (maybe add spears, halberds or lances?)
Just wondering aloud...