How about specific architectural styles? One thing that lacks in Simutrans is definitive, era specific skyscrapers/mid-rise. You ask for something in the modernist style, built between 1960 and 1970 or something like that. Or a contest featuring one specific city's skyscrapers or landmarks.
Some other ideas (category, then some examples):
Ancient land vehicles (chariots, carts, rickshaws, wagons, etc.)
Ancient or early industrial ships (galley, caravel, side wheeler, paddle wheel river boats, early screw types)
Exotic or unusual way-types (tram roads, pole roads, corduroy road, plank road, narrow/broad gauge rail, etc.)
Geared locomotives (Shay, Climax, Heisler)
Depots for various way-types
Special extension buildings (oil, gas, goods, logs, bulk, etc.)
Power lines (perhaps older, early 20th century types?)
Future or conceptual vehicles
Just some ideas, some may not work so well though..