If you get into the realm of speculation and the hopeful yet far-fetched, there are plenty of candidates for pre-de Soto Europeans setting foot in the US. There are even monuments to them.
1.) A lot of people think the Vikings made it down to New England. AFAIK, no respectable historian believes this. But there’s actually a statue of Leif Ericson, in a rather stubby little Viking boat, on Charlesbank West in Boston. That’s part of the green lawn separatinmg the two halves of Commonwealth Ave. to the west of Massachusetts Avenue (or, as Bostonians put it, between the halves of Comm Ave to the Fenway side of MassAve.)
2.) Other people think the “Newport Tower” was built by Vikings, or by an Icelandic Bishop. It’s in Newport, RI.
3.) Others think that Prince Henry Sinclair landed in Massachusetts (after visiting Canada) in 1398 and buried one of his men in Westford, MA, off 495 to the northwest of Boston. They’ve put up a chaqin perimeter around a rock they believe to be engraved with a memorial to the knight vcarved, they hold, in 1398), and there’;s now a granite marker explaining all this.
4.) The Dighton Rock in Masachusetts has carvings on it, now well-weathered. There have been various claims put forth as to who did the varvings, including Indians (for a change), Irish monks, and Vikings. One of the more interesting claimks was that it was done by Joao Vaz Cortereal, brother of Miguel, before Columbus.
5.) There are lots of others – Welsh Prince Madoc was held to have come over in the 12th century, and there’s a monument to him in Kentucky (!), or the Kensington Rune Stone in Minnesota, carved b Vikings who penetrated suspiciously far into the US without leaving marks elsewhere, or Mystery Hill in Salem, NH, claimed for Irish Monks and Vikings and God knows who else. There are tons of other claimants, but these fascinate me because somebody believed them enough to pay for and erect markers for them sometime in the last 200 years.