Just make the whole thing free-to-play. Sub prices are too high for me to get involved in this nostalgia. It just doesn’t make sense to pay a subscription to an abandoned game from almost two decades ago. The subscription for Shadowlands only was $5 a month, which I believe is a more reasonable sub cost compared to $15 a month.
Not only this, but the server will be under populated if you require players to pay a sub. Most players own froob accounts for mules and buffs as well as their main paid account(s).
I think that the progression server should just end with Shadowlands content. Do not progress to Alien Invasion, Lost Eden, or Legacy of the Xan. Later expansions only power crept gear to a point that made existing content trivial in both rewards and gameplay. The over-powered gear and perks released in these expansions homogenized gear setups and lowered the range of player customization available rather than adding to it.
You said on your stream that you would be interested in player feedback regarding how long the servers should last and when expansions should be released. I would ask that you be very cautious before releasing AI/LE/LotX, and make sure to ask the players if they think adding the expansions would be good or bad for the game.
Consider that battlestations from LE drew players away from the pvp arenas. The alien gear from AI removed countless items from all level ranges from viability while putting everyone in the game on an endless viralbot grind. Perhaps it would be good to rethink these sweeping changes and reimplement them differently. Such as perk lines that determine the best weapons for a profession and pigeonhole players into a specific playstyle.