Why exactly is Poland being absorbed "natural"?
I mean yes, there was German settlement there, but that's not the same as the duchies being drawn into subordination to the Emperor.
I certainly think it's possible for a HRE which is focused on eastern expansion, in the right conditions - but it's not going to happen just because German culture is so awesome the Poles just want to be part of the HRE so as to get more of it.
If you ask my opinion, with a stronger (more centralized) HRE and/or weaker (more fragmented) Poland than OTL, the highest probability outcome ("natural" in this sense) is for a bigger chunk of Poland to get
culturally and politically assimilated by the Empire, by the same parallel processes that happened IOTL, only enhanced in these circumstances.
As it concerns Bohemia, Silesia, and Greater Poland, IMO butterflies would have to really go out of their way to prevent their complete absorption in the HRE, and snatch defeat from the jaws of victory, so to speak. It would otherwise happen without need of any serious focus on eastern expansion by the Emperors ("natural" in this sense, too). Masovia would require a somewhat more serious purposeful effort by the Empire, or the right dynastic/diplomatic/military butterflies. Lesser Poland is a halfway case, it might go both ways, depending on butterflies.
Hungary and Croatia are like eastern Poland, but only rather more so. Their
political assimilation is feasible, but it requires a rather serious Imperial focus on eastern expansion, and/or the right dynastic/diplomatic/military butterflies, which also need to involve whomever rules in Constantinople acting in the right way.
It is also most likely that those areas would get kinda more
culturally assimilated than OTL, quite possibly in a patchwork way, as circumstances make the same processes that affected Transylvania and Dalmatia more successful.
It's not (mostly) an issue of Imperial culture being so awesome, rather of reciprocal strength. However, I may point out that a centralized HRE would inevitably be one of the most powerful and wealthy European states, and its prestige would be proportionally bigger. As things typically went in premodern European powers, power, wealth, and prestige would eventually translate into a sizable degree of cultural supremacy, as scholars and artists flock to the Imperial court, the Emperors and magnates give them patronage, and so on.