Growth Dependent Block Size Limit
Technically, this is possible. However, it would be a hardfork, so all users would have to upgrade. In such a large ecosystem that exists around Bitcoin today, this would be rather disrupting.

Such change would also have unclear security implications. Miners would be naturally eager to accept as many transactions as possible, so a spammer may gradually push block size limit up to increase the time it takes the network to process new blocks. This would lead to an increase in orphan rate, which would make it easier to mount a 51% attack, especially with the help of selfish mining.

This idea was discussed many times on Bitcointalk. While most of the developers agree that a change would be necessary eventually, any specific plans have yet to be announced.

By the way, some altcoins, such as Monero and Cryptonite, have variable block size limit using rules similar to the ones you propose.