The Modern Key Dates..

The year that will someday become a key date is 2009. The mint only made about 10% of the number of coins made previous years, not to mention the Lincoln's Life pennies. I've been looking for a while, and I found my first 2009 dime just the other day. They are getting hard to find even now, so hoard all you can!
Happy Hunting!

If they actually ramp up the dollar coin production in 2013 as it was recently proposed, the 2012 dollars could end up as keys to their respective series. It's, of course, all relative though; the 2012-D Chester Arthur dollar has a mintage of 2.8 million, and the 2012 Sacagaweas aren't even the lowest mintage in the series (that would be 2008).
Of course, all golden dollars except possibly the earliest ones are for a long time going to be rare in lowball grades (i.e. Fine and under). They barely circulate after all

S Mint circulation strike ATB Quarters are another low mintage coin. They may be readily available for some time into the future, but if demand is higher than supply, the price has to rise. A coin might have 2 million mintage, but if you have 5 million collectors that want that coin, price has to rise because there aren't enough for everyone.

Are you looking for the next 1877 IHC? 1909sVDB? 1916 SLQ? etc. Because if you are, I don't think you are going to find it. If there was a readily available modern coin that had big potential to be very valuable down the road, people would hoard it making it not so valuable. i.e. the coins you pointed out. Given the modern mintages, I believe we are past the era for "key dates" equivalent to the widely known classics. I believe the focus with modern coins will be on the highest grade you can find, i.e. the 1983 Quarter.(except for coins like the 2011 25th Anniversary ASE set, which immediately jumped in price due to collector demand and the mint's short sightedness on mintage figures). I would recommend that you collect what you like and not worry about the investment aspect. There are much better investments than coins.

The JFK silver proof is a record low mintage. The only one that beats it is the SMS 1998-S Matte Proof.

I'm guessing by key dates you mean like the 16D, 21, 21D, Mercury dimes, for example. Or the 1909S VDB, 14D, etc. Lincoln Cent. Coins that are necessary to fill an Album and are rather on the difficult to find kind. My opinion to that is NO.
The quantity of almost all coins minted lately are rather excessive to ever be considered a Key Date coin. Yes some may grow to fame a bit like the 2009 series, but none will ever be a Key Dated coin. Yes there are those that discuss the errors, odd balls, etc. but with some exceptions, most never make a slot in Albums. Naturally there are those dumb exceptions like the 22 plain Cent, 55 Doubled Die, etc. that for some reason Album manuafacturers think we need to complete a series.
Again, for the most part I don't forsee any moder coins ever being considered a Key Date.
One exception might be the 1964 Nickel. So many people are sick and tired of them everywhere, someone may start a massive melt down of those.

Depends. Are you willing to wait a few hundred years to see how they go up or down in value.