These are species which may possibly be recorded in future. Of course, predicting what will occur next is mostly birder's fun, but includes also much educated guess. Some of the possible additions may appear improbable. However, since I first made for myself such list several years ago, several X species occured in this country. Let's wait and see, what will be next.
Most species are on the Western Palearctic list.
However, some are not. The rarest Siberian vargants are usually first recorded in Britain and Holland, which are at the opposite end of Europe but have most active twitching communities. Of course, proving, that some of these species occured naturally will be very difficult. This concerns especially strays from naturalised populations abroad and waterfowl, falcons and passerines common in captivity. I must say, I thought that Pied-billed Grebe and Vireo in Poland were rather strung.