Personally, i think it makes a lot more sense for a number of reasons. 1- Both IPs belong to Bungie, so they would have a lot more creative freedom and knowledge of the universe they're working with; 2- It would generate content for their main IP, while also bringing more people to know and potentially like another one, so they wouldn't risk taking resourses away from their main selling point to work on something else entirely. Even if they still wanted to release a new Marathon, at least there would be a bigger playerbase potentially interested; 3- Technically, there already IS a Marathon crossover within Destiny. Mida Multitool's lore and design hint the weapon came from another timeline, Marathon's timeline specifically. So why not go all in with the alternate timeline lore thing? Maybe bring more weapons with the same premise; 4- Marathon is PvPvE, so is Gambit. Why not use this opportunity to give the most despised gamemode in Destiny's history a renewed focus? Think about it