Good news, Professor. Model Shipways has announced a new kit of an 1880s Ohio River sternwheeler, and from the photo in their catalog, it looks very credible. Its good to see the Western Rivers represented by such a typical vessel, and will be in wood, given its source. I'd add that its good to see new quality wood models by American manufacturers, too.
A kit of Delta Queen (and Delta King, which has been restored minus its engines to its 1920s appearance in Sacramento) would be popular with modern steamboaters and Americana buffs, and it would sell. A Hudson River style sidewheeler like Mary Powell would be sensational, as would a workaday sternwheeler like the ones up in Alaska or the Northwest. And a more regular Western River sidewheeler would be pretty neat too.
In terms of "Must Haves," using the "available in plastic" and "worth building" criteria, here's my list, using my preference for early vessels:
Zvedza Cog
Revell Mayflower
Revell Batavia
Vasa
Maybe the HMS Prince but at 1670, its pretty modern. I think there is a kit for a 1620s French ship but I cant remember. Soveregn of the Seas is not typical, so I omittted it. None of the Santa Marias are worth bothering with, and there are no good carrack models out there. But at least we have a really great cog, a nice Elizabethan merchantman, a pair of large Dutch designed vessels, and with the Prince, a ship of the line.
Right now I'm building the resin Dutch yacht of 1746 from Artitec and its sensational. Way too modern for me, but I really wanted to build one of their kits. After that is Skuldelev 3 and then who knows.
Jim