The two small islands just off the west coast of Demacia look like they could be volcanic in nature (funny how there is a trail of smoke what it looks like coming from that island pair).
The two island chains bellow that one could have split from Shurima at one point (as their shapes fit the upper left area of the Shuriman Continent coastline)...but they can also be Volcanic in nature.
There is a giant Hawaiʻi looking Island piece on the far right corner of the large map. That looks like it could be a piece of land that got split from Northern Freljord at some point, and by some chance has elevated central mountain area (which could also indicate of a dormant Volcano, or tectonic plates pushing the land upwards...or it could be magic).
The rest of the small unknown islands surrounding Valoran and Shuriman Continent look like they split apart from the mainland from ether rising sea levels, or tectonic activity.
And no, there is no lore for any of those unknown islands as of yet. This is just my guess based on the visual appearances of said islands.
As of the know Island/Continents: well it seems like Ionia could have slit off from Western Valoran at one point (but it also has islands that look volcanic in creation; also a lot of plate tectonics to push those central mountain chains up); Valoran and Shuriman Continent slit in the are where the narrow sea is (Piltover and Zaun was the last area where they were connected before Progress Day happened); Bilgewater looks like an entire Volcano made Island chain, with dormant volcano islands and a cavern system left from its creation (possibly magically altered); Shadow Isles looks like it could have split from the corner of Shuriman Continent between the Icathian peninsula and Southern Jungle coastline...or it was made naturally by volcanic activity.
Valoran itself has visible scars from the Rune Wars (Noxian coastline has a giant Rune War explosion crater coastline; also its has very little vegetation) . It also looks like the area between the lower Valoran Mountains and Upper Valoran mountains will split at one point, since the Mountains themselves are going upwards in separate directions (funny how there is an entire open field area that doesnt allow Demacia or Noxus to be fully isolated geographically). Freljord seems to be entirely geographically isolated due to large mountain chains, glaciers, fjords, glacial lakes, and permanent snow coverage due to being near the poles (This is why Demacia and Freljord have more isolationist type of communities, its all in their geography). Shurima has a similar issue but its near the Equator. Surrounded by mountains (giant af Mount Targon) on its western part due to converging tectonic plates, its completely desertlike in the center, East doesnt have that issue and is why it has lush vegetation. Also the Mother of Life river has a clear connection point to the Serpentine River. Icathian penninsula is just dead and entirely cracked due to the void.
