Surprisingly, according to these FCC filings, Kobo intends to release a 7.8-inch e-reader - the Kobo Aura One. If accurate this would be the largest e-reader released by Kobo. The question arises on what Kobo's objectives would be with a larger e-reader? First, the extra screen estate would be useful with comics and graphic novels.
The more obvious use case scenario would be PDF files but both Amazon and Kobo develop e-readers for purchased e-books, with PDF support an after-thought. To be fair, Amazon does allow, with most non-scanned PDF files, the possibility to highlight, take notes and utilise dictionary support. Kobo's PDF support, on the other hand, is merely a bare-bones PDF viewer, with little functionality. This means, if the FCC filings are accurate, Kobo would need to significantly re-vamp their PDF software, if they envisage users to make use of the extra screen size.
Another possibility, with the extra size, is to offer choice at the premium end of e-readers, against Amazon's recently released six-inch Kindle Oasis. If E-Ink Carta is ready for the larger size then it would put the new Kobo e-reader at advantage, if both e-readers are priced similarly. From the end-user perspective, this might mean, finally, Amazon responding with a larger Kindle.
If a 7.8-inch Kobo Aura One is released and Kobo decides not to enhance PDF support, it might be possible to install KOReader. KOReader, when stable, is a feature-rich e-reader application that supports both e-books and PDF files. However, to rely on KOReader for PDF support means the end-user would need to be proficient enough to install the application and resolve any possible compatibility problems.