Whenever I see this request, I have to wonder what people mean by "harder".

Are we talking harder, longer boss fights akin to trial fights or alliance raid bosses, with a lot of precision mechanics and instant death mechanics? The problem this runs into is the game is largely built around the 8-man party, with at least 2 party members with raises. Even competent players can die due to mistakes or lag or DCing or whatever, leaving a party of 4 SOL if the healer dies and there's no RDM or SMN. I've had otherwise pretty routine dungeon bosses wipe simply due to the healer dying due to a mistake, and not all tanks can self-sustain themselves. Look at the one four-man extreme trial we have in Rathalos EX. They had to introduce specialty mechanics to compensate for no raises. The alternative would be to give more classes raises, but seeing as SMN came very close to losing their raise in 6.0, that seems like an unlikely direction they'll go.

Mob packs are tricky, because we don't really have hard mob pulls in this game. The best example I can think of is the long pull after the second boss in Anamnesis Anyder, but that's not really difficult, it just takes a marginally competent tank/healer duo to manage. Without adding a bunch of randomized nonsense, they can only increase difficulty of trash by adding more mobs or by making them hit harder, but that just boils down to more of the same things we already have. I can't really say I get a great deal of joy out of trash pulls, personally.

The only ways I could think of that wouldn't be completely annoying or tedious is either to make 8-man dungeons, which wouldn't really replace the current system at all and would be it's own duty type, or to add a logos/lost action system, but I feel like before too long those will be solved systems, essentially removing the difficulty with cheese strats, because that always happens.