I have mixed feelings on this.
In a prequel we usually know for a fact that there is a preordained ending shaping up, and that something bad will happen to the characters if they seem to be going against the ending we know will happen. In that way a rather gloomy sense of inevitability might permeate the series as potentially new characters are introduced, only to be stomped on and crushed while we see them struggling against inevitable corruption by Sauron or Sauruman. It is difficult to build characters up to viewers only to see them fall to the inevitable.
The other problem this creates for story writers is that with already established characters you have to take pains to keep their actions in line with the original canon if you want your whole storyline to make sense. In the Hobbit films it makes some sense for a shout out to Gimli to be present, as its a one off no one would likely remember. However, say if in this series you suddenly had Gimli and Aragorn meet and interact, it would be impossible for them to not remember that in the Fellowship film, and would just take a steaming dump on the original canon of the books and film if that were the case.
My fear is that there will be many liberties taken with this, in a way that there were numerous unadvised liberties taken with the Hobbit films to stretch it into a trilogy. One would hope that the Tolkien Estate and the writers of the series will do their best to simply make this a faithful new addition to the Lord of the Rings filmography. Otherwise we could end up with opinions as bad as those of the Star Wars prequels.
On a personal level I would feel far more comfortable if they instead opted to tell contemporaneous stories within the original trilogy. The War of the Ring had many fronts, and they could easily be explored within a television series. Heck, the story of the Blue Wizards could finally be put to paper and adapted faithfully. There's loads of stories to tell in Middle Earth, one needs only the will to write it.