First off: I love Wondrous Tails. What a great way to get players old / new to play older content. My idea would feed off of that core idea. Here it is:

Over the years, many challenges have come and gone. Over the years, many players were not able to experience these challenges (maybe they were inexperienced at the time, busy in real life, had not started playing FFXIV, etc.). My idea would be to resurrect these challenges.

Imagine that one of your Wondrous Tails weekly challenges is to take down Twintania (BCOB 5). Now imagine that when you click on the challenge you are given some additional background: "Twintania was introduced in patch 2.0 that was released on August 24, 2013. At the time of its release, the highest possible item level was item level 90 with a level cap of 50."

I would propose to keep Wondrous Tails as it currently is; but with a small tweak. Thus if you beat Twintania (under any circumstances) you still get a seal. However, if you beat Twintania under the original difficulty (level 50 / item level 90) you get some sort of small additional reward and your accomplishment gets added to a "Time Capsule" log. Thus the player will have a log that shows the challenges he / she completed under their originally intended conditions. Perhaps down the road there could be rewards that are based on the quantity of these "Time Capsule" accomplishes that a player has accumulated.

Just throwing this out there. Nothing more. Would be easy to implement. The main purpose of this idea is to provide incentive. Without incentive; the likelihood of getting 8 people together to defeat Twintania under its original difficulty would be slim to none. There are some ridiculously well crafted challenges in this game that are now long forgotten. I'm just looking to provide "purpose" to these challenges once more and resurrect some great content that gets lost in the vertical progression of FFXIV.