There are essentially two types of sequels. One would be the continuing story sort, where, although the novels stand on their own and have their own arcs, there is an overall unifying plot. The three novels of my Malvern Trilogy fit that model pretty well. Maybe the four Donzalo’s Destiny books do also, though it might be argued that it is one large novel (or, conversely, a series of eleven different novellas and novelettes). In that each Donzalo book was published before the next was written, I feel it is appropriate to call them sequels.
The other would be an whole new story for our protagonist(s) — or maybe, a secondary character moves up to the lead for the new narrative. The three novels in the Mora Trilogy (which can itself be seen as a sequel to the Malvern Trilogy) go this route. There is an underlying plot element, to be sure, but not really a continuing story. However, there is not a clear line between these two kinds of sequels. They may just be a bit of each.
My latest, ‘The Jewels of the Elements’ (out February 2 of next year), tends more to the latter. It is billed as a sequel to ‘The Eyes of the Wind’ and shares the plot element of the mystical jewels known as the Eyes. It also continues the adventures of the noblewoman Marana and her now-husband, the craftsman Saj. But it is a new story with a new plot and a new setting. There is no overarching plot unifying the two books.
Eventually, there will a sequel — or sequels — to the Donzalo’s Destiny books. Donzalo will not be in its pages that much; rather, the focus will shift to Lady Fachalana. That is, the story will center around her as the previous one did around Donzalo, but there will be plenty enough subplots. When I might start on it, who knows? There are other projects waiting. Some of them just might be sequels.