XI-2? Did I miss an entire FF MMO? LOL

I'm not sure how you'd make this work with the Duty Finder. Would you queue for whichever raid size you wanted or would the DF version be fixed at whatever size is the "base" size for that raid? Flexible instances were used in FFXI in a few places like ENMs and Assaults, and things like Dynamis had a window for group size but didn't scale anything based on that. The deterrent to going in with max group in Dyna was simply a dilution of the drops across more players. With things like ENMs and moreso the current Delve, Skirmish and Incursion content, the "flex" simply amounted to a pile of extra trash monsters and some beefed up NMs which only lends itself to "flexing" your team by adding more DPS. This could damage role balance quite a bit for tanks and healers. Plus extra trash leans teams towards more AoE based DPS (BLM BRD).