Someone said Elisif would be the most possible choice since surrendering under Ulfric means he might indirectly make Elisif into a ruler on behalf of Skyrim.
She remains the
official Jarl of Solitude if Stormcloaks win the war doesn't she? Who knows, a few years after the war, when things calm down a bit, Ulfric might want to forge something of a political marriage with her (which could win him former Imperial supporters). If this happened and then Ulfric died before his heir was born or of age, it would open a gate for the Empire to sneak into Skyrim again, since Elisif as we've seen her so far is incapable of ruling independently. This could potentially lead to another civil war because a lot of people would hate to see Stormcloak victory go to waste like that.
The only way to prevent such a scenario would be either Elisif growing some decent cojones while ruling on Ulfric's side or Elisif marrying someone who can actually rule immediately after Ulfric's death.
Other than that, the only other inheritance options I can see are:
- Ulfric marrying someone who's strong-minded enough to rule without him,
- Ulfric remaining single and being superseded by one of his main generals,
- Ulfric, regardless whether married or single, dying in a duel and being replaced by the victor.