More Dungeons like Fork Tower from FFV.
Imagine in Pharos Sirius was 8 man, with two parties climbing up a split path. Party A faces an endless wave of enemies while party B fights a mini boss, once party B wins, the waves stop and party A can advance. So on until they reach the top where both parties reunite to fight Siren.
Wish all the new dungeons were like that, since they're using "iconic" bosses. Siren and Diabolos could have been closer to the Primals in difficulty if the dungeons were 8 man. Not to mention it would make dungeons feel much larger, since they'd effectively be doubled in size. It also would mean that you'd need to do it at least twice to see everything, rather than seeing everything your first run.