I find appealing the idea of a Field Exploration instance with Carnival elements. However, rather than having field action, it would be more appropiate if there was an array of instance related items that worked along BLU toolkit (for example, tokens to draw Mimicry buffs from). The exploration part would merge well with a class that is intended to be played to explore other content. You could even open more venues for spell learning (and, obviouslty, new spells). My only issue with the idea is the limiting factor that the Blue Mage poses on the content, as it gates one kind of content behind a different kind of content that doesn't necessarily align in term of interest (people might be interested on FE instance but abhor Blue Mage and viceversa)

A Deep Dungeon for BLU doesn't really ring with me, on the other hand.

However, the take some people here share on making BLU a real class is completely terrible. The only appeal BLU has is literally being able to customize your spell list. It doesn't have any kind of gameplay loop standarized classes do and trying to bake Blue Mages into a normal class will leave a chimera that will be dead on arrival at the cost of killing a content that at least appealed to some people. Might as well ask for a new class altogether instead of deleting content just because. You don't like the limited class? Don't play that kind of content. Easy as that. This is the same kind of egotistical train of thought that also ask Bozja-like content to stop coming just because forum dweller didn't ask and SE clearly has to tailor content to their specific tastes and no one else's. Stop trying to take away content from others who actually like that content.