Being a wife of a king and emperor was certainly more attractive than being a wife of a minor duke, so Elisabeth would prefer the Bohemian king. But assume it is impossible; which Masovian duke is she supposed to marry? Siemowit III? One of his sons, Janusz I or Siemowit IV?
Siemowit III - I'm not sure, he might be a widower at the time, so he might be available. IOTL his daughter married Casimir III's grandson, Casimir IV, so Casimir III should not be against this marriage. Although it would probably make the marriage between Casimir IV and Siemowit III's daughter (which happened in 1369) out of the question.
Janusz I is of the same age - a most likely candidate, IMO.
Siemowit IV is 5 years younger, besides, he is a younger son - Janusz is more likely candidate.
If she marries Siemowit IV, it doesn't change so much - both his successors IOTL, Janusz and Siemowit IV, are already alive (they were sons from his first marriage); IOTL Siemowit III had a second wife, but his only surviving son from that marriage became bishop of Płock. OTOH, he would be a great-grandson of Casimir III.
If she marries Janusz, it also doesn't change that much: Janusz was a loyal vassal to Poland and Anjou dynasty, he probably remains the same.
If she marries Siemowit IV - it might strengthen Siemowit's position when he tries to get Polish crown for himself in 1381. If Siemowit IV has a son with Elizabeth, he can claim he fights for the crown not for himself, but for Casimir III's great grandson. Personally, however, I do not think it would work. The decisive voice belonged at the time to the noblemen from Małopolska (Lesser Poland) since they controlled Cracow and had strong support from Hungary. And they prefered Jogaila. Siemowit IV probably acts the same he did IOTL, with probably the same result - he fails. Altough his chance might slightly better, so who knows?