Part of me thinks that it'd be a nice way for people to be able to experience the items that represent Gaia's past, specially since many old users came back due to nostalgia. Some, like the OMG Hat, the Angelic Halo or the Demon Horns are quite iconic and representative of Gaia's history, and if I remember correctly some items (most notably the Angelic Halo) have been previously re-released in one way or another. People deserve to be able to enjoy them.
But part of me thinks that re-releasing them as-is wouldn't level the playing field for everyone involved. Allow me to explain.
I remember the days when MCs were the backbone of the Gaia economy. Investing in stuff is always a gamble and many of the Letters and MCs I hoarded since the late 00s are pretty much worthless nowadays, but there was a sense of "safety" knowing that there was stuff that wouldn't lose it's value (or at least not completely). It's a promise that wasn't guaranteed for even basic things like the old Gold or the new Platinum, for example. And it was fair for both ends of the transaction: Gaia was a relatively small business in the Wild West days of the Internet which could use the userbase's money to keep afloat and flourish, while the users (many of which were on the younger side and couldn't buy things online as easily as nowadays) would get cool items that in a pinch could be sold off for a gold headstart. After MCs, other items that could be valuable enough to be worth holding onto were things like high-end EIs, or certain event items (back in the day, Halloween 2K5 items were top-of-the-line for this, but the Resistance Mask from 06 or the Durem Shirt from the 07 Water Festival also apply).
But nowadays the backbone of the economy is a different one. Things have changed a lot since the old days, new items are released incredibly frequently for either very high prices or RL money, and for the people who want to get items that aren't available on the MP anymore re-releasing MCs would only solve a very small part of the issue. There's almost 20 years' worth of history on this website to sift through, and many of those items will never get a chance to be released again. I sure don't expect to get a Water Meat anytime soon, that's for sure.
And all of this is not even considering that what will probably happen is that if the items get released once more, people will buy them en-masse, then keep them in their inventories until they're scarce and valuable once again. Old habits die hard, after all.
The solution? I can think of a few.
* Make them identical-yet-separate items, in a similar way to the different Generations for Evolving Items, so that the original ones can "keep their value" for those who want to play a numbers game instead of enjoying the items for what they are. If someone wants to have an original '04 Ninja Band because it's more valuable than a '24 Ninja Band then they can chase after it, but if someone wants to cosplay Kakashi they can go after the new one rather than the vintage one. Worst case scenario, all you have to do is add a release date to the item description and that'd be enough to make them distinguishable from one another.
* Do what you did with the 2008 Anniversary Sashes, the Potatoverseer and Sentinel skins, or the zOMG rings and make them soulbound/account-locked. In Team Fortress 2 it's possible (or at least it used to be possible) to buy certain cosmetic items as they are, for a reasonable price, and without having to gamble using resources (keys or metal). They cannot be traded to other users or used for crafting, so they're strictly used for cosmetic reasons since they have little to no value in the community market.
* Make them slightly different. You already tried this with recolors and have seen that they work wonderfully, if the twelve dozen Ancient Katana, Demon Horns/Tail and Ninja Band knockoffs have anything to say about it. This is less than ideal, but it's been tested already and we know it's serviceable. Plus, you can make slight adjustments to them that might not have been possible in the past (such as adapting them to newer avatar bases or adjusting color schemes).
This is strictly regarding MCs. Thank You Letters should stay out of the equation completely, as they were originally meant as a way for Gaia to thank it's users. Nowadays this isn't a donation rewarded with an item, it's a standard microtransaction/purchase and the middle step is completely unnecesary.