We are already very far from original satoshi idea... so I propose to use a floating threshold. If no one is able to find a block in 2 mins then threshold will be increased. Or better say that new block threshold should be somehow linked not only to the previous 2000 blocks and separation between them but also to separation between this block and previous. It is hardly to find any hole in there... looks fine to me at first sight. It may introduce some kind of oscillation but I do not see any big flaw that can be used in attack. If you are issuing new block in just one second then your solution shall be shorter than say 5 second. If you are issuing next block after a five minute of game than your solution can be very slow and not optimal.
How can you reliable measure time interval between adjacent blocks? I don't think there is a way.
The sum of block times in a chain shall be always smaller than the current time. It can be used to let miners to build chains where block will be accepted if it contains a solution passed the threshold deduced from a block separation from previous. For example if a new block is issued in K mins from previous then the solution shall be faster than the square of K. If you want to issue a new block in 10 sec then your solution shall be shorter than (10/60)^2 * 60 = 1,67 seconds, if you want to issue a new block in 30 sec then solution can be already 15 sec long. At one minute mark you will be able to issue one minute long solutions. Then to build a longer chain in the same period of time you will still needs to build it from rather optimal solutions. You wouldn't be able to make a long chain of block whose solutions are inefficient because floating threshold will prohibit it.