Odd!
We were discussing something highly similar in another thread,
Changing and rearranging SC, where
Rose-tree Dryad was putting forth some useful ideas concerning "The Seven Friends of Narnia" and how they could be used to remind audiences of characters from other stories. Such as by using "Friends of Narnia" meetings.
At the end of SC Jill goes to a party where she wears her Narnian clothes. Someone suggested that Eustace escorts her to this party - or meeting - saying "there are some people I'd like you to meet". I've also my own ideas about how this party might play into how the rest of the films could be done and who might be present.
The remaining films, whichever are chosen, might be described as the Eustace and Jill show for SC, the Polly and Digory show for MN, and maybe the Aravis and Shasta show, if it is done at all. A side story in LB is how the Seven Friends of Narnia plotted to go back to Narnia, using the rings Polly and Digory planted in London in 1900. Either in the credits at the beginning, or better still at the end, a short scene where Eustace at such a meeting asks questions to the Professor, Peter, Edmund and Lucy, such as "Who was the White Witch everyone talks about"?, would be useful, perhaps. There are already references leading up to HHB in SC, the book, such as at the banquet Eustace and Jill attended, and the tune Prince Rilian was whistling, as he returned to Narnia, after he was disenchanted. And Professor Kirk, as an adult, is the one whose story leads into LWW, and the Pevensie story which includes HHB.
A trilogy sounds a bit better than talking about sequels and prequels since the Chronicles of Narnia are Narnia's story, and the human individuals whose stories are told aren't necessarily related to each other. They aren't even of similar ages and Polly and the Professor would have seen two World Wars by the time the series finished. Shasta and Aravis don't even get into the real world, leaving Edmund with Lucy's help to tell their tale if any questions arise. Lucy, after all, was a friend of Aravis. The first three books published were the Pevensie trilogy, but SC is the final book of the Caspian trilogy. The remaining 3 books are just miscellaneous. Unless one groups together VDT, SC and LB and calls these three books the Eustace trilogy.