If Poland didn`t oppose to Hitler (not with him, nor against him):
* Casualties in lives of ethnic Poles in WWII would be less but, Poland would have greater casualties inflicted by Soviets, at the end of WWII and even later
* Poland wouldn`t lose its Jewish population
* After WWII, Poland would face partition. Internal antagonisms in Poland, among Poles themselves, would create two strong factions. One would suggest solidarity with Russians (Soviets) over Slavic connection and, on the other side, other faction would force resistance to communism. Even if wasn`t directly involved in WWII, let`s say by some miracle, all the time would Polish society face this dilemma- Slavic solidarity with Russians or/and fight against them due to communism and historical animosity.
* ultimately, two Polish states would be formed, comparable to Western and Eastern Germany.
* Western Poland would be stricty monitored by western Europe and would be among founding states of EU
* Eastern Poland would unite with Western Poland and united Polish state formed, same way as it was case with Germany
* Considering old and newer history and having in mind truly strong Jewish element (for sure more then 4 millions) in that new Polish state anti Russian feelings would be much stronger then in today`s Polish state. In fact, Poland would absolutely belong to western Europe. If we say that today`s Poland have chance to resist to assimilation by western Europe (if nothings change dramatically) maximally about 100 years, that hypothetical Poland (of which we talk here) would have less then 50 years of capacity to resist to assimilation by western Europe and, with it to stop to be considered as Slavic.
* In our time, considering global today`s mega trends, most probably that would rift within Polish society arise to the maximum (as i mentioned- over division Western/Eastern Poland/strong Jewish element) and it is quite possible that would, as consequence, Poland face another partition but this time not on two but on at least three or four states. Two main territories would be Polish (Western and Eastern Poland) then, one territory would be declared to be Jewish European state and Silesia would secede (only eventually Silesia wouldn`t secede from Western Poland due to strong support of western Europe and USA to Western Polish state).
* i wouldn`t exclude possibility of Civil War within Western Poland, when would assimilated Poles tries to deal with still stubborn Slavic Poles that remained living in Western Poland. As result, Slavic Poles would be forced to go to Eastern Poland as refugees and eventually final border between Western/Eastern Poland would be formed.
* As a result, all what would (in this hypothetical scenario, 50 years in future) become part of Western Poland, Jewish European state and (?) Silesia, would entirely lose any connection with belonging to Slavic world. Only Eastern Poland would remain Slavic. Actually, with time would Eastern Poland remain only named as Poland (so, that hypothetical Polish state would be smaller then it is today in our reality). Rest would acquire some other name, to even with it cut all ties with its Slavic past.
* But, as i contemplate, what is interesting, that hypothetical Poland (what remain of Poles) would eventually (due to historical experience) have bigger capacity to initiate formation of Intermarium, then it is case with today`s Poland (from our reality).
* Intermarium would be created