As Patrick O'Brian is on record as having read the Sharpe books, I imagine that you will be familiar with O'Brian's Aubrey/Maturin series. The series became far more popular than O'Brian thought they would be, and he ran out of Napoleonic years. His solution was to create extra years: 1812a; 1813b and so on.
I wanted to ask whether you might consider the same device for extending the Sharpe series? 'Assassin' has him in Paris in 1815; there appear to be hints from both Wellington and our hero that he will be available if called upon. Rather than casting back to the Peninsular years for the next novel, what a joy it would be to see Sharpe promoted to full Colonel (or General, which was held out as a prospect to Sharpe at the farewell dinner...) and recalled to the Colours in 1816 etc.
Any glimmer of a chance?