Personally I build reasonably simple boats but of various designs, tug, MTB, speed boat, launch, Rig Supply vessel to varying levels of detail.
I range of hull designs with interchangable superstructures / top works would enable the intermediate modeller to build one hull to the scale of their choice (by printing off the plans to the size they want eg A5 =1:48, A4 = 1:36 A3 = 1:24) and pick the style of top.
Later they could choose another top and build with more detail, then by repainting the hull there is a "new" boat !
Planing hull = speed boat / fast river cruiser / naval patrol boat / MTB
General work boat hull = simple day fishing / harbopur utility / trawler / tug
Obviously they would not be true scale representations of real ships or boats but could be decent semi scale projects.