I love the idea of having other podcasters come on, either in an interview format, to join the discussion, or whatever. Something I think that makes it interesting is that (I get the impression anyway) some other podcasts sort of view the DIS as a bit of a black sheep. Certainly, none of them rise to a level of open hostility--or anywhere even close-- but it's definitely apparent, and occasionally gets mentioned.
Usually, it's just some criticism of the moderation/filtering that takes place on the DIS; while many of us who have been here a while understand some of the reasons behind it (or even if we don't, we accept it as the price of admission), it's understandable that it may seem inappropriate or heavy handed at times.
Getting more to the point though, I think possibly one of the most interesting guests is actually one of those individuals who has criticized the DIS at times. (I don't think that'd get in the way of the friendship among disney lovers though, and certainly wouldn't act in an unprofessional manner)...
One of the things that makes DU stand out a bit from a lot of other podcasts is that it openly puts forth constructive criticism, and they've said this. This other individual is possibly the only other one that I can think of who takes a very similar view (and in fact, pokes fun at having to listen to the "happy fuzzy bunny" reviews). This podcaster also has a strong understanding of Disney history, legacy attractions, and the like, which makes it even more interesting; but isn't local to Orlando or Anaheim, so still has the "vacation" perspective. So, in spite of any perceived dislike of the DIS, it's exactly that individual who I think could be most interesting.