As a Fantasy or Sci-Fiction writer, it is of no doubt that you may have considered the possibility of either multiple dimensions or worlds within your story. While some manage to do this naturally, others struggle with the concept. So then one begs the question, is it even worth it? That answer is up to you and should be considered carefully.
What is the advantage of a Multiverse within a novel?:
The answer is quite impressive, to say the least. The best part of a multiverse is the sheer potential for spin-off stories, fanfictions, time travel or alternate realities. These alone provide you with more source material to continue writing past your first novel, and for your fans, they may just as well love it! But then there is the cost...
What is the disadvantage to a Multiverse?:
When approaching the concept of a multiverse, you now have to put the effort into setting each one up. Keep in mind that a multiverse does not mean multiple planets, but refers to multiple universes or realities. Each with its own unique timeline and rules. Maybe one universe allows magic but the MC gets stranded in a universe that does not? The possibilities are endless, and they are entirely up to you. It could be as simple as specific characters not existing within different realities, or so big that the entire course of history is different!
So what is the advantage to a Singleverse?:
Truth be told, simplicity. A singleverse is easy to manage, easy to work with and requires the least effort. It is still able to provide material for spin-off stories and expansion, but it won't be nearly as fruitful as a Multiverse. Granted this can also be taken as a disadvantage, depending on your POV.
So what is the disadvantage to a Singleverse?:
In my opinion, the lacking potential for original content. You will hit a point at some time in your life where you just start to repeat yourself. This can become dull for an audience that has followed your work for the longest time, or, will be accepted the second time around. Not likely a third time around though.