If it already exists in the game and I have missed it, please disregard.
Right now Hearthlings max out at a 6 in their Mind, Body, or Spirit stat. My suggestion would be for some sort of quest that would allow you to obtain an item that would give you the ability to buff one of your hearthlings by a point. Generally I’m talking about Body or Spirit because once a Hearthling is a master at a profession, Mind is secondary.
Most of my interaction is done with my soldiers so I’m a little more partial to them. The ability to make very rare investments into their abilities would only raise my attachment to those characters. Not to mention it would be useful to have more powerful troops as the game goes on, rather than just continually growing the army.