If I were to guess, one of two things might happen:
1) Queen Elizabeth II will likely be replaced with whoever the reigning monarch will be (King Charles, or whoever that reigning monarch may be); or
2) To allow consistency with other denominations of banknotes, the monarch may be replaced with a Prime Minister who has never appeared on our banknotes before. Probably someone with historical significance that is fifth in rank.
I'm betting on #1 happening.