Alex maps should be stackable. You are limited in terms of how many you can acquire, and the Alex themselves stack, as someone pointed out.

As for the other treasure maps ... sure, make them stack. The only change that will make is allow people to save them up to run them. It may increase the rate at which they enter the game (right now, I doubt everyone is gathering a map daily, whether to use or to sell) but the overall effect would be to decrease their availability on the marketboard and increase their price there. That's it. And it may not even do that! Sure, people will be able to form a party and run more than 16 maps ... but how many people are actually forming 8 person Full Parties and running 16 maps right now, anyway?

Making the maps stack would be a QoL thing more than anything else.