Hypothetically, if the future Albert I died young (before his accession in 1909), and more specifically before the birth of his first child (the future Leopold III, in 1901), who would succeed Leopold II?
Speaking as perhaps the only belgophile in Philadelphia....I've seen a family tree somewhere but I can't find it now. There must be one on Wikipedia. Salic law was in effect until the early 1990s, so look for Albert's next-youngest brother, if he had any; otherwise you're into brothers of Leopold II, or failing that Leopold I.
EDIT/UPDATE: Okay, you piqued my curiosity.
Here's the family tree:
http://en.wikipedia.org/wiki/Kings_of_Belgium_family_tree
Albert's only brother, Baudouin, died at 22 in 1891 and is not shown here to have married. So there is no one eligible in that branch of the family after 1891. We're only in that branch of the family - descended from Leopold II's brother Philip - because Leopold II himself had no eligible descendants: his only son died at age 10.
Since Leopold II and Philip were the only sons of Leopold I to live longer than a year, it appears that if Leopold II dies between 1891 and 1901, Belgium's got a problem. They can't (I suspect) go into descendants of Leopold I's siblings (because the throne was offered
to him and his descendants at independence). They've got to legislate that, or legislate something - repeal of the Salic Law, offering the throne to a son of a daughter of a past king, offering the throne to some random unemployed foreign prince.... What I'd be wondering at this point is how ornery Flemish nationalists (or Walloon nationalists - they existed too) or anti-monarchists were being. Brussels is already mostly French-speaking at this point so splitting up the country wouldn't be much easier than it would now; Flemings and Walloons can't just draw a neat border and go their separate ways....
EDIT/UPDATE II
On re-reading my answer, I realize "if Leopold II dies between 1891 and 1901" wasn't your question. But the answer's the same if Leopold II outlives Albert and dies with no other heir. Although in this instance, Parliament has time to act, assuming said Flemish and Walloon nationalists and anti-monarchists aren't being too ornery.