If Victoria had died before William IV, the crown would have in all likelihood passed to Ernest Augustus - the fifth son of George III (George IV was the first and oldest, William IV was the third, and Victoria's father, Edward, Duke of Kent, was fourth.)

As luck would have it, Ernest August inherited the throne of Hanover anyway, so it the succession had bypassed Victoria then both kingdoms would have remained in dynastic union.

In terms of keeping both crowns joined, there was no discussion of locking Victoria out of the succession that I'm aware of. There was talk of setting Victoria up with Ernest August's son, George of Cumberland (the future George V of Hanover), so that the kingdoms would remain associated and then be rejoined if Victoria had a male heir.

Victoria and George got on very well, but the idea of Ernst August having any sort of influence over Victoria was anathema to parliament, William IV and to Victoria's possessive mother.

In 1810 Ernest August had been found with a dead valet in his bedchamber - the circumstances remained unclear, but rumours circulated that the Duke had murdered him - and he had been far more politically active than the government liked, leaning on George IV and being accused of unfairly influencing the election of Weymouth's MP.

Plus, George's inheritance was no sure thing anyway - the prince was blind and there was debate in Hanover as to his suitability as a result.

Ultimately it's worth remembering that the British government had no real interest or stake in Hanover - and there was a great deal of suspicion about and resistance to Britain getting dragged into continental affairs on account of the Hanoverians - so the driving force behind keeping the crowns united was with the royal family. The British government had no incentive to upturn the succession, and were perhaps even relieved to be shot of Hanover.