Mandates can be fun as a challenge at first, but they rapidly get boring. There are several reasons for this. Partially, nobles tend to spam the things they like over and over again in mandates -- it isn't a problem to satisfy them (unless like something something really weird), but it's repetitive and eventually gets unrewarding.
Which leads to the second problem -- you don't really get anything from mandates. I mean, yes, they're a challenge, but I'm not even talking about fabulous rewards -- there's just no payoff to the player in terms of making the game world more interesting. You complete a mandate for a suit of platemail, you end up with a generic suit of platemail, just like every other suit of platemail you ever made. Boring.
So here's my suggestion: Replace mandates with 'commissions' that are almost like mini-artifacts. Instead of simply producing mandate for a generic item and throwing it at the player, the noble chooses a specific dwarf with the right skills, holds a meeting with them, and assigns them a mandate to produce their desired item.
This item would be a 'normal' object in most ways (it wouldn't be unnaturally high quality or made of strange materials or anything, like a real artifact), but the noble would include requests for engravings and decorations, as with an artifact. The assigned dwarf would do all of this. They would produce the commissioned object like a normal job, but would take longer than usual, and would be treated as having a slightly higher skill than they actually do for that one job (dwarves are honored by commissions from nobles, and put more effort into the things they request.)
Most importantly, when viewing the completed object, it would note that it was produced as a commission for noble XYZ, by dwarf XYZ. This would help work commissions into the history and legends of Dwarf Fortress, especially if the commissioned object itself ended up being used for important things -- a sword that was commissioned by the king is something interesting, and it shouldn't just get dumped into the pile of generic swords and become unrecognizable. It should have some fancy decoration on it, at least.
Dwarves would get a small happy thought from having a commission assigned to them, and another for completing it; they would get an unhappy thought if they failed (which would have the penalties it does now, although obviously the dwarf who gets punished would be the one it was assigned to.) Nobles would perhaps try reassigning the job a few times if it got dropped or interrupted -- maybe they'd even try a different dwarf, though they'd get madder and madder as they do (this would be helpful if the dwarf who initially got it assigned was injured, say.) Once / if their temper finally blows, of course, they assign punishment to all the dwarves that failed them.
Commissions would perhaps be somewhat rarer than mandates are now (because they're likely to involve more materials, what with the decoration requests.) They'd also be somewhat harder, for the same reason. But I think it would be interesting.