Maybe 30-45 minute boss fight might be a tad steep. I do agree with your idea in general, I don't want content focused mainly on challenges, I want it focused on heavier difficulty of the content itself. The ogre and Batraal are ridiculously easy to tank, assuming everyone around you knows what they are doing. Leave challenges for the achievement system. I think to balance out better gear, I would make it based on rarity, not how fast you can get it. The idea of letting you back in anytime is a great idea, but making speed run content gives you less opportunity to enjoy the dungeon.

I would actually like a large scale raid like WoW's, where there are several bosses and a major boss at the end, all difficult, requiring strategy on each one, no time limit, and we can scale through it at a reasonable pace. Like say there are 12 bosses, your LS takes down 4 of them one night, but everyone is tired and doesn't feel like going through anymore for the night, so we continue it again the next day. You have a weeks worth of time from when you enter the dungeon to complete it, after that, it resets. Its difficult, would help cater to casual players. There would be some tweaking to be done to cater to FFXIV's system, but something similar to this would be a major approach for the game, and I am sure it would be approached more positively than negatively.