I don't think action sack is going to happen for a while. Halo 5's forge and custom gametype system are sophisticated tools for the devs and community, but they've yet to maintain a solid role due to a couple of things:
343 very recently implemented a file browsing system on Waypoint, allowing players to browse and download commuity creations Before, people had to find the creator's gamertag and friend them to get the map. While this is a step in the right direction, not everyone uses Waypoint; until 343 implements an in-game file browser, players will struggle to acquire community content.
343's Community Cartographers, first implemented in Halo 4, are in a controversial position where some cartographers are allegedly abusing their power to obtain influence within the forge community, target individuals they don't like, and promote friends' maps at the cost of other forger's (potentially better) content. One name comes to mind in particular: Psychoduck. However, it's best to refrain from accusation without significant proof.
Nevertheless, community creators struggle with promoting their content before it can be downloaded, and few maps outside of the Com. Cartographers' inner circles have made it into BTB, despite being creative, viable maps.
Action Sack won't happen until changes are made for content sharing. MCC benefited from porting earlier gametypes, but Halo 5 is a new engine with new tricks, and it's going to be a while before players feel encouraged, or at least comfortable, in creating playlist gametypes and maps if 343 takes decisive action.
Community content is at the heart of Halo 5's longevity; 343's free DLC won't be enough for players when Halo Wars 2 comes out.