Yeah, shoehorning in a romance plot will only hurt the film, just like it hurt Prince Caspian. The ONLY Narnia film that could have a romantic subplot without just totally ignoring the source material is The Horse and His Boy since Shasta and Aravis DO end up getting married eventually. Even then, it would need to be kept very subtle as half the point is that they DON'T like each other at first. But you could easily show them having confused and conflicting feelings for each other while staying true to the source material.
Then again, probably the main reason Eustace and Jill aren't a couple in the books is because they're really young. They seem to be barely teenagers at the time of The Last Battle. So if they keep Will Poulter (as I hope they do), they'll be casting Jill in her late teens or early 20s, she'll almost certainly be fairly attractive, and they'll be playing the characters as 16 or 17 years old. At that point, anyone in the audience not familiar with the source material WILL be asking why on earth the two aren't a couple, and not at least addressing the issue could be seen as a glaring omission.
With Jill and Eustace playing 16 or 17 year olds in The Silver Chair and potentially 20 year olds in The Last Battle, it WILL have to be addressed at some point or else it will be a little silly. If I were the screenwriter (And hey, maybe by the time of "The Last Battle" I will be!
), I'd handle it like this:
The Silver Chair is basically the beginning of Jill and Eustace's friendship, and especially early on there is quite a bit of friction in the friendship. So you really could get away with not showing any romantic feelings between the two in The Silver Chair, and instead focus on developing their friendship and showing the two develop a mutual appreciation and respect for another.
By the time of The Last Battle, though, several more years will have elapsed and the two will have been best friends for some time now. In that case, what I would do is weave in a small subplot about Eustace's interest in Jill. It could start with Eustace looking longingly at Jill while the friends of Narnia are having their meeting, only to be noticed by Peter and Edmund who snicker affectionately at him leaving him embarrassed. Small things like that - little glances and lines of dialogue - could keep reinforcing the subplot while they're in Narnia, and as the story progresses perhaps Eustace could eventually confide his feelings in Tirian.
Maybe Tirian could notice the way Eustace watches Jill as she trains in the distance with her bow while they're staying at Tirian's forest outpost and broach the topic. He could tell Eustace that Jill is a maiden worthy of pursuit, and that it would be a shame for Eustace to die in the battle that night without confessed his feelings to Jill. Later, as the Battle seems set to begin and things seem grim for our heroes, Eustace could reach out and take Jill's hand, and she will clearly be thankful for it.
And that would be about it. Enough to address the topic, but subtle enough to be respectful to the book. What do you guys think?