I would like to see dogfighting (the aerial kind, not the furry kind) made into a civilian sport. In training exercises they're able to simulate kills without any live ammunition being on the plane, so the technology exists to do it... kinda safely. The main problem is the expensiveness of the hardware, that F-15E I posted costs something like $50 Million, a cheaper F-15A will run you "only" $30M or so, new. Used, you're looking at $10M or so, but you have to buy them from the USAF and something tells me you'd have to fill out a couple of big scary documents.
So obviously, it'd be a sport only for the very rich, or for use by renting planes from a private company for a fee. But I believe you could make a fairly decent subsonic jet for $1-2 million if you make it small and use off-the-shelf technologies. I think a 6G-capable airplane would be feasible for that price, and of course flying one would be such an amazing thrill that I'm sure some people would gladly pay thousands for a day. I would. There are already services that let you rent a brand-new Ferrari for a day, so I think this could really be seen as sort of a big version of that, and of course you could also charge for BFM and ACM lessons, licensing, and even simple rides. And considering that fighters are generally built to survive continued heavy service for 30 years, the long-term profitability of such an organization may well actually work.
Interestingly, the Russians made a trainer-jet called the L-39 Albatros, which you can buy (used, of course) today for less than $300,000. Performance is a little wet; just 0.8 mach and 36,000ft, but it just kills any car you can get for the price. I don't care about your 0-60 time, what's your 0-600 look like?