you want some good reason of why we don't have open world dungeon?

- if it's the best place for get some item, it will be over farmed, you will have more player than monster and the purpose of the place will be totally lost.
- if the place is not really interesting for the item, people will desert it, it will be content that only a few people will do and take place on the server. and it will be the inverse of the previous case, it will loose it purpose since no one will come there.
- finally the best, if you have quest there to do, you will have either the first case or the second. in the two case the quest will be a pain to do. (especially if it ask to be done in group)

RMT are too a trouble, people say rmt will not farm monster, they have done it and must still do it. open dungeon it's a pain, in term of balance, space for the server and purpose.