I think time and manpower play into how much time and effort gets dedicated to that sort of DLC. With the Headhunter Packs, there was more than one side quest in most. They weren’t long quests, but in each there were two or three or so other quests/ activities you could do.
The weird quirk was that in Bloody Harvest one of the Quests was hidden with secret objective. Wattle Gobbler’s Grandma Flexington quest is something I don’t think most people had the patience for, and there were two missions in that quest line. Mercenary day did skimp on the additional quests & activities beside a repeatable loot farm, but given the theme, looting as a focus seems appropriate.
The Wedding Day Massacre had the most things to do, with the Leprechaun, the Threshers, and Innuendo Bot’s quest and Son of Crawmerax had the Sparky Mission line, the treasure syringe, and the Raid.
I like how Dahl Headlands is a fun map where you can flow through multiple missions at once, after they’re all active. It makes it feel like a truly open world, because we’re not locked into much a linear flow of missions once the bounty board is up and running. The Dust & Lynchwood have that vibe, and so does Triton Flats in TPS.
To be fair, most of the main areas in Borderlands (1) are set up that way if you grab as many available missions as possible as you progress. It was the DLCs for Borderlands (1) that really pushed the linear mission flow of doing a story mission, turning it in, and unlocking side quests for that area. That has always been pet peeve of mine. Borderlands 2 does it in fits and starts, with there being some missions you can access while completing a main mission for an area or the story, and others being inaccessible.
How much of that is by design due to the need for a character to be on level for the next story mission in NVHM and TVHM rather than the game scaling with your character in UVHM? Is it just a matter of missions having to be rationed?