The Catholic Church and the Church of Jesus Christ of Latter-day Saints (the “Mormon church”) are two mutually exclusive entities in multiple regards. It simply is not possible for there to be “unity” between the two, if by unity you mean the sort of unity that people think about between Protestants and Catholics and Catholics and Orthodox.
In all cases of “unity”, there is the implication that one or both groups will have to somehow modify certain doctrines or beliefs so that the unity can exist. For example, for unity to occur between Protestants, Orthodox, and Catholics, there would have to be a change in understanding of ecclesiology. The problem with “unity” among Christian groups is that each tends to believe that they are right and have the fullness of truth. To embrace unity, they may have to accept that they are wrong on certain issues.
The reason why unity between the LDS Church and the Catholic Church is much more problematic than unity between Protestant Churches, the Orthodox Church and the Catholic Church is because of [at least] two foundational issues: the Great Apostasy and the nature of God. Mormons believe that the original Church founded by Jesus Christ lost its priesthood authority, which was then restored by God in the 1800s. Catholics believe that the original Church has always existed, and that it is it (Orthodox believe the same thing about themselves).
This difference prevents any sort of unity, unless by unity you mean one group giving up their beliefs to just accept the other group’s beliefs. The LDS Church has no point in existence if a Great Apostasy did not occur. The Catholic Church has no priesthood power if a Great Apostasy did occur.
The nature of God is also a problem, since Mormons do not believe in the Trinity. So, for unity to occur, one group would have to say that their understanding of who God is is wrong. Mormons believe that their understanding of God is a “restored” concept (which also includes newly revealed points), which goes back to the Great Apostasy issue. Either it happened or it didn’t. So, the issue of who God is is very problematic, and of course is not something that Protestants, Catholics, and Orthodox encounter in such dialogues, since they all accept the Trinity.
So, there really isn’t any hope for doctrinal unity (and/or intercommunion) between the LDS Church and the Catholic Church, especially since each rejects the other’s priesthood authority and power, and each has a “one true church” claim that excludes any possibility of the other being the “one true church”. What there is hope for is continued work on social issues that are common between the two.