Attaching a number so that more than just the very last piece is Imitated; for example I2 for the last two pieces. Could be modified using lower-case letters for things like only the last two opponent's pieces.
Allowing for a Protean; that is, a piece that assumes the powers of the last piece it captures. Maybe could be done as cI or xI. (It could also be a modifier for M; cM could be the basic Protean, cM3 for something that imitates the last three pieces it's captured, etc.)
I recognize that these might be more easily said than done, to the point of not being doable at all, but it's a thought.
Two possibilities for the I (Imitator) atom:
I recognize that these might be more easily said than done, to the point of not being doable at all, but it's a thought.