In terms of add-ons, character fits etc. the DAZ Morphing Fantasy Dress seems to have been really successful. Maybe make Something flexible and adaptable like that, whether clothing, a car that transforms from a hardtop to a convertible etc.

I vote for a pixie too. Not that I can afford it but I couldn't find a classic 40s-50-s "Woodie" wood paneled station wagon except 1 at da Squid. 

Collections of things to fill out a scene. There are military uniforms but I sometimes see people looking for a messkit etc. You have to look everywhere for the little items. A picnic basket with all the food utensils, blanket etc. Things like Terkkie's 'Trash Street', a set of modern medical instruments and accessories for a doctor's office. People keep asking for 'everyday' clothing but if they're doing everyday scenes there are a ton of little things. I don't recall seeing a car with a romovel tire ( and a flat tire morph), a working jaxk lug wrench etc.  Maybe those things are too specialized but I think simply having them could inspire a scene.

 

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken