A "hub and spoke" raid of 6-10 bosses.. Minimal but challenging trash leading to 2-4 bossfights pr. "spoke".
Dear god, yes, this. My absolute favorite instance of all time in any game was Naxxramas: 15 bosses in a single instance, minimal trash between each (at least in the Wrath version; the classic version had way more trash than was actually necessary, but that was largely because it was designed as a massive time sink that would take 3-4 raid nights over the course of a week to finish). It felt massive without actually requiring monumental amounts of travel time. It was *great*. Ulduar was a close second because it did much of the same.
I understand why the developers have gone with linear operations (the operation follows a story rather than simply being a location that you are assaulting) but it's not as if they couldn't have each "wing" treated as an independent story that, when you complete it, is referenced in the conversation/dialogue of wings the next wing that you take down. It would also mean that they could actually have each boss drop 1-2 pieces of loot and actually provide an acceptable amount of loot for the course of an entire raid (HM TfB and NiM EC are just painfully stingy) without giving everyone all of the tokens they'll ever need super quickly (first bosses in a wing only drop belt or bracer token and offset boots or something of the kind, second bosses drop a token for a "better" slot and an offset item, etc.).