At some point in the future there will be a wipe. XP and Crafting xp are the only safe items currently. However I think we need to think about the national economics of some of the things that could be wiped from the game.
Resources, Ships & Materials- Wiping out these would really hurt the current economy. One of these wouldn't be so bad, two of these would cripple, and all three would probably lead to some players just putting the game down till everything gets normalized for a few weeks, Currently there are ship queues for several of the larger ships extending out weeks and these are occurring even with the slow and steady rise through the ranks. This will even out soon enough, but if all of these happen at the same time then we really have a problem on our hands. The time it takes for the devs to increase the production of buildings and decrease the production of ports slowly points that they understand the need to take care with the economy.
Gold- Not really an issue as its really easy to make.
Production buildings and upgraded warehouse- Not a huge problem if these are wiped. If gold is wiped at the same time then there is a small problem.
Port Wipe- Wipe if you want to but I think we would just be back at the point of the game where are all nations are expanding back out to there respected population. Mass expansion time was a fun point in the game though, but I kinda like that all nations have fit there size for better or worse and fronts have been created. Production buildings do make having multiple ports less necessary. The diversity and proximity of those ports to your capital matter a little bit more.