Precisely, it would be as if the transactions in the most recent block are actually still in txpool.... not confirmed until they get some blocks behind!


Ah, no, this would not be the limit the search space... this would simply be a way to give humans more wall-clock time to find their solutions, at the cost of overall network integrity.  For example, if N is 4 then you need 4 times as many confirmations to be sure of a block, but a human miner then also has 4 times the average bot solution (wall clock) time in which to find their solution.  Forced map resets would be 1/4 of what they are currently.

The fact that we (arguably) drop the network integrity by 75% (really we just defer the generation of that integrity by an additional 4 blocks) is probably not much of a concern considering the bots will likely be adding hashing strength by a factor of at least this.

Finding the right balance for the value of N to make a good tradeoff between security and human-mining might be tricky.

This is not an ideal solution, but it is a relatively simple/straightforward one. (If it can be made to work, and if the community is willing to live with the extended confirmation times.)  I worry some about the fact that it certainly does weaken security, but I'm not sure if this is a rational worry: the current security threshold is practically "who knows" because it is not like we have good (or any) analysis of the integrity of "motohash" algorithm, so what exactly is 1/Nth of "who knows" security, and is 1/Nth good enough?