The current idea to be implemented sometimes soon is as follows:
Between 966 and 1040:
If Polans become Christian, control Greater Poland and some other provinces (no great prerequisites there) they become Poland, with historical cores. If they control less provinces they become country "Greater Poland". This country, along with other Christian Polish states (Masovia, Silesia, Lesser Poland which is created out of Vistulans) have the possibility to become Poland at every time if they are strong enough and control area of Greater Poland. Clearly, Polans and Greater Poland have the highest chance there.
Auto-annexing other Polish minors is probably not planned (far too easy and it took around 25 years for Poland to historically do that) but some events in the case of vassalship are an interesting proposal. At least having cores on these regions should help to get them militarily. And Silesia, Vistulans and Masovia will convert to Christianity eventually and the fact that Poland is Christian will contribute to that.
After 1040 the situation is similar but it becomes a bit harder and Krakow is required as well (which becomes capital).
After 1138 it changes, it hasn't been scripted yet, but an intricate system of managing fragmentation period is in "early design" phase