From what I have seen in terms of values coming from Mundus stones, some of them tend to be underwhelming to some, if not many, but then I thought, why not leave it as it is, but in addition to granting the second Mundus Stone effect, it would also make the mundus stones twice as effective while it is being worn?
For an example, lets say a mundus Stone grants 100 spell damage, but if twice born star were to be worn, it would X2 the effectiveness of the Mundus stone & make it 200 instead of 100 while wearing the set.
Something like this could make twice-born star more viable & more worthy of being a 9-trait craftable, but the question is though, wouldn't you want something like this?
If you see me anywhere. Know that I am sitting back with a bag of popcorn, watching as ESO burns the goodwill of its player base with practices that only disrespects the players time like it did to me and many others...
If a game does not respect your time, best thing to do is move on from it and find something else.