This could be implemented as a soft fork too.

* 1MB hard size limit

* 900kB soft limit

S = block size

U = UTXO_adjusted_size = S + 4 * outputs - 3 * inputs

A block is valid if S < 1MB and U < 1MB

A 250 byte transaction with 2 inputs and 2 outputs would have an adjusted size of 252 bytes.

The memory pool could be sorted by fee per adjusted_size. 

 Coin selection could be adjusted so it tries to have at least 2 inputs when creating transactions, unless the input is worth more than a threshold (say 0.001 BTC).

This is a pretty weak incentive, especially if the block size is increased.  Maybe it will cause a "nudge"