Casimir's chosen heir at the time of his death was Casimir IV of Pomerania. I do not see the reason form him to change his mind.
Władysław Opolczyk would be about 60 in 1390, which would make him too old and he had no sons, which would cause problems with succession after him.
Siemowit IV was a possible candidate, since he had some support in Great Poland and Kuyavia.
A lot depends of what happens before Casimir III's postponed death.
Actions of potential heirs would influence their chances to actually take Polish throne. They might gain or loose some popularity and support among Polish nobles. Casimir IV might fight in the war for Silesia and die or cover himself in glory or shame, or not fight at all; Siemowit IV might support Poland, support Bohemia or remain neutral. Władysław Opolczyk might have a son, instead of only daughters. There might be someone else Casimir III deems worthy of being his successor. 20 years is a lot of time.
P.S. When Casimir IV died in 1377 he was only 26 years old, and he was mortally wounded in combat - with Casimir III living longer Casimir IV's death is most probably butterflied away.
And the Anjou dynasty is out - they run out of male successors in 1387 and that according to Polish-Hungarian treaty from 1355 eliminated them as candidates for Polish throne. Not to mention Sigismund of Luxemburg (the king of Hungary since 1387) was a brother to Wenceslaus IV of Bohemia obviously defeated by Casimir III in the war for Silesia. Of course assuming that all the births, deaths and marriages in royal families of Central-Eastern Europe happen as IOTL - which probably would not happen.