Usually a country, or sovereign entity of international law to be more specific needs some area to claim over that sovereignty. This is a problem the Tuvalu nation might be facing in the near future.
While it doesn't seem much to lose a nation with a land area of 26 square kilometers (it's the 4th smallest country by area), they could be offered a similar track of land somewhere else to occupy, the land actually comes with some other benefits. Like the rights to the sea areas around that land including fishing and mining rights to whatever lies beneath the surface. In that sense Tuvalu is not that small anymore. By this exclusive economic zone Tuvalu is actually 38th biggest country.
If the matter of owning an area of dry land is the defining factor maybe Tuvaluans could build a platform on top of one of their islands to keep their sovereignty. Sure that wasn't enough for attempts in creating new nations like the one on top of the old offshore platform from the second world war. But maybe in this case it would help that they have already been recognized prior to losing their land.
But there is also one sovereign entity today that also hasn't had any land under their reign since the 18th century after the French occupied Malta. Sovereign military order of Malta is an interesting case that warrants it's own post.