Personally I like having a tier split up between different raid instances. Having smaller chunks to deal with allows groups to do things in chunks and vary what bosses they hit if they hit a wall even if each raid is linear. The split also makes hitting the last boss of a raid faster and allows a group that is doing progression to clear a few bosses instead of having to make the loot sacrifice to extend the lockout in order to give the group time to progress on a boss. If a group does decide to extend a lockout then they are missing out on far less potential upgrade chances if they do such. With smaller raid zones they are overall more casual friendly with less time commitment which translates well for PuGs. By also having separate raids it allows PuGs to do one raid at one point and still have another raid to look forward to another night instead of having a one shot commitment to raiding for the week that can end up bad and leaving them sour until the next lockout.
Another thing that could be done is just splitting up the lockouts of raids like LFR is like. Perhaps even make the wings separate like the Scarlet Monastery model that the developers did say they liked yet never brought to raids. It might feel weird and players may skip whole wings though it is just like having separate raids and the theme can remain the same instead of coming up with new art work and lore for a whole separate raid zone. This allows overall more flexability and casual raid groups can more readily find a replacement if they have already cleared part of the tier as they can start another zone and still progress instead of trying to find a person willing to throw away half the tiers loot table.