Sven Co-op II
Sven Co-op II was originally announced on July 3rd, 2003 (https://www.moddb.com/mods/sven-co-op-ii/news/svenco-op-team-announces-sven-co-op-for-half-life-2) as a sequel to the Half-Life mod Sven Co-op. Although Half-Life 2 was yet to be released, planning had begun, and as part of the announcement Valve also "expressed interest in helping the SvenCo-op team following the release of Half-Life 2" (https://www.moddb.com/mods/sven-co-op-ii/news/svenco-op-team-announces-sven-co-op-for-half-life-2).
Throughout 2004 and 2005 a number of updates including screenshots were posted on Mod DB, but the project soon went quiet. The project was officially cancelled in 2008, with developer Adambean saying in 2018 "Sven Co-op 2 was cancelled in Summer 2008. We're not interested in revisiting this project." (https://steamcommunity.com/groups/SvenCoop/discussions/0/1694920442945872600/). Although maps and models were created, none of the assets were ever released.
