I think what is missing is good guide about "how to improve your gameplay" written by someone experienced. Actually, there is no guide of any sort which could help new players understand what is important in some areas (especially area like blackfire), what is their job in a team and so on. That in combination of playing usually worked great for all other games I played.
As a solo player, I must say playing solo and playing in a team are two different things. Your playing role is completely different. In solo play you must pay attention to dps and keeping yourself alive, in team you could pay attention only to healing, only to dps, to things you are designed to counter, to watch out for teammates, to control your aggro and so on... Guide is, in my opinion, best tool that can be given to new players so that they know at least a theory on what is expected from them.
So far game has been quite single player friendly and it does offer challenges regarding farming and killing. Hill Giant is great learning spot for basic boss fight mechanics and is good enough for that I think. You should pay attention on what you are doing even with well farmed characters. Granted, with some it is very easy fight, with some is a bit challenging, but a point is Hill Giant is soloable with all classes and is probably done by everyone to farm crystals (they probably kill him in a team, yes).
Whitepine area is grinding stuff for solo play, but crafting bags in combination to rare spawns, you can actually well farm at least some of your characters without ever stepping foot in a multiplayer, minus trinkets you must play blackfire as a team for. Rare spawns are much more challenging than HG and I'm not sure are they soloable with all classes.
There are already a lot of learning aspects and challenges for a solo play. I assume point of a dungeon would be to force players to play solo and not in a team, but, you know, that kinda misses the point that gaias is designed to be multiplayer game and that multiplayer and solo are different. For example, bishop is amazing solo farmer (possibly best?) and is played as dps character with heal backup in solo... completely different than what his job usually is in a team. Someone playing him solo will not really prepare him for multiplayer playing.
I don't think new solo dungeon would change anything or offer anything new what is already not in a game. Perhaps there could be new stuff to teach players for example to take dispel, and not that silly meteor, but that is a long shot. Great guide masterrace IMO, though we must embrace the fact there are plenty of players who simply want to play the game and not necessarily play it efficiently.