I don't have my roadster yet, so I just have to sit around and think about it.
Some of these I've seen examples of on other cars, some are not realistic, but here goes anyway:
1. A sliding hardtop. You reach up, undo a latch, and slide the hardtop back. (Yes, getting out of the car to put on/away the soft top IS a pain when you live in a place with really unpredictable weather). Ideally it would slide down out of the way, but I don't think there's room for that...maybe it can go straight back, and you can still see out the back window under it.
2. A replacement "wagon-style" trunk lid. Maybe it could mount and latch in the same places as the current trunk lid, but it sits up higher, so there's more height to the trunk and you can even put stuff on top of the battery (ideally things that don't retain a lot of heat . It could have plastic or glass on the sides and back for visibility. Maybe the sliding hardtop can somehow motor itself under here.
3. A hitch. I'm sure somebody will use it to haul a generator, but I'm more thinking of a bike rack. Something really easy to remove (ha, ha) so I don't have to carry the extra weight around all the time.
4. Rather than a whole car cover, just a cockpit cover. So you can park it with the top off, and quickly pull it from (handwaving commences) some semi-permanently mounted position on the back or side, and secure it with a strap. It would reduce interior fading, keep out bugs, dust, leaves, and ideally even rain.
5. Photovoltaic body panels. Yes, I know you get very little power that way. It would still be cool.
6. Floor mats that are plush, stay put, and don't curl. And don't get pine needles stuck in them.
7. Inductive charging--install it on your garage floor so the car starts charging as soon as you park over it.
8. A CD that plays rumbling ICE noises. Better yet, something monitoring OBD to generate appropriate sounds for conditions...with a quick and easy shut-off button.
9. Motorized 30mm cannons. Always fun in heavy traffic.
10. Grab handles.
11. 110V outlet in the trunk (if it only works when the car is NOT running, that's fine). It should have a "Do Not Use With Hair Straighteners" sticker to keep my daughter away from it.
12. Remote control via cell. Get notified when charging is done (or stops!), put in to valet mode before giving key to spouse, listen over microphone, start the stereo, etc.