I think you should be encouraged to group with other people--not necessarily pug--to tackle difficult content. This is one of the core tenets of an MMORPG and it doesn't need to change. You know what the old adage is about what ain't broke. Someone above pointed out that DF (and pugging, by nature) is something of a lottery. You can't make difficult balance for a pug to take down. You need a group of people you can rely on.

Now, a flexible raid size would not be a terrible idea. Require at minimum the core number of people required to complete the mechanics (maybe a light party) and scale boss HP and damage according to how many people you bring up to a reasonable number of people.

Of course, that sort of thing brings with it its own challenges for the dev team. Imagine having to balance so fights designed for a group of 4-8 don't get unreasonable for two alliances.

As far as lockout goes, I think future raids like Coil should just work off of the Crystal Tower system. You can run however many times you run for appropriate soldiery+poetics rewards, but upon winning loot--are locked out of loot once per week.

But that is just my two cents.