No specifics as it's more of an abstract idea, but for something a bit different or maybe a themed or budget project how about taking a reasonably elaborate model or miniature railway and either upscaling it to full size or actually creating it in a model railway setting? It has been done in Trainz - even to the extent of having a substantial catalogue of overscale items to represent the railway room (or even bigger the house and other items for a garden railway). Also a couple of times I believe in Classic TS. Could give plenty of scope for intensive operation if the design was chosen carefully but in a contained area so need not be too heavy on the file size or performance overheads.
I like completely out there type of ideas like this one, shows people aren't bogged down with sensibilities, i once posted an idea that what if the broad gauge in Britain had survived and we had modern trains but as broad gauge that would be a interesting route and DLC.
