Danny on TowersStreet Talk has made an interesting discovery within Zierer's IAAPA product magazine that arguably drops a heavy hint towards Legoland's new coaster being a Zierer family shuttle rather than a Vekoma Family Boomerang, and also provides an insight into a potential theme for the new ride:
I don't know about any of you, but that "Dueling Reverse Coaster" looks exactly like Legoland's proposed layout to me, and the cops and robbers type trains in the left image could possibly hint towards a Lego City theme, in my view. That would certainly suit both Lego and a duelling roller coaster, so would work well, in my view.
P.S. Here's Danny's TowersStreet post: https://towersstreet.com/talk/threads/legoland-windsor.67/page-77#post-382844