The trouble with forecasting future collectables is that as soon as someone makes that prediction, everyone will buy one and put it away. Thirty years from now there will lots of mint condition whatevers available and the market for them will be zero. You see so many things, gunz, dolls, toys, etc. that are marketed as "collectors editions" that are basically worthless in the future.
Find something of top quality but not rare that will be well used/abused by most people; in the future if you keep it in top condition it may become collectable.