If I may suggest something; do not go on a crazy WC campaign, as it will ruin (IMHO) the splendid atmosphere you have created. Manpower can be increased by agricultural technologies, and I'm afraid annexing your neighbour will result in net reduction of manpower, as you won't be able to win without taking loses.
Since the Axis is gone, I suggest creating a block of allied countries between the Baltic and the Black Sea. At first glance, Hungary or Romania look as potential fascist enemies, but without Germany, there's no-one to put antagonise relationships between them and Poland. Hungary and Romania have always been most keen on Poland (the Hungarians not only refused to participate in Fall Weiss, but also treated Polish refugees and PoWs with respect - vide general Maczek and his Armour-Motorised Cavalry Brigade). Speaking of Romania, someone has already mentioned the alliance against the Soviet Union. I guess that Transnistria concept can be revived.
Italy is far away... But still can be useful, because of Mussolini's hatred to communism. I can easily imagine him supporting Poland against Stalin. Don't forget that the idea behind fascism was not "we support Hitler because he is cool" but more like "we support Hitler because with him we can wipe the reds out".
The Balkan question is most interesting. Serbia was a natural ally to Russia - but an alliance between a communist country and a monarchy may be possible only is extreme situations (like RL Yugoslavia in 1941). On the other hand, alliance between Hungary, Yugoslavia and Romania simply won't work because of mutual territoria claims. There are two possible configurations, IMHO:
1) Alliance with Romania and Yugoslavia, possibly even with Greece
pros: Romania will support you against the Soviets, while Yugoslavia and Greece keep Hungary and Bulgaria checked should any of this countries try to conquer their claims
cons: possible war against Hungary, Bulgaria or Italy
2) Alliance with Hungary, Bulgaria, possibly Italy
pros: a strong ally in the South, with some valuable blueprints and resource, interesting perspectives should Italy send expeditionary forces
cons: basicly, you will be alone against the Soviets, at least for some time. Possible war against Romania
And the Baltic states. Historically, Lithuania and southern Latvia (up to Dvina) were parts of Poland since the Union of Lublin (1569) to the Third Partition (1795). They can be either incorporated by force (which can serve as a pretext to leave the Allies), or considered Casus Belli against the Soviet Union in addition to Belarus and Western Ukraine. Estonia and the rest of Latvia can be either released as an ally, or considered conquered lands in accordance to Zygmunt III's pacta conventa of 1587 (these were promises that a newly elected king of Poland had to make before coronation; he promised that Swedish-controlled Estonia shall be incorporated into the Commonwealth. Their execution in 1599 sparked the first Polish-Swedish war).
