Mabinogi shouldn't be a tanking game but rather avoiding hits. Part of what made mabi exciting before was when party members were a must and everyone needed to hide in 1 corner.
-only 1 aoe effect of a pet can be activated per minute. Ex. summon 2 ice dragons, only first one will have effect
-Mobs need instant skill load and agro
-Mobs need to do 10x higher damage than they currently do
-The common mobs need newer skills (fighter, gunner, lullaby, shock, crisis). How about shadow gunners that spam shooting rush?
-all player damage should be cut to 1/2 of what it is right now
-need more crisis/freeze/lullaby resistant mobs
There is always "harder" dungeons that has some of those above properties. However, they aren't efficient, and they take too much time to complete. (Some take too much effort to even enter)
With your suggestions, who is your targeted audience/player? It definitely isn't the new players. Is it the mid level players? Is it the super old and high level players?
And then people go back to simple mode Hard mode Shadow Missions or something along those lines. If monsters deal 10x the damage, it will put more people into deadly more often, and then they can just pot back up.
The truth is... Mabinogi is an old game. Some of the game mechanics are still slowly being upgraded to fit today's modern MMORPGs. The barriers to start playing this game to get to mid-game is pretty high in terms of time and/or money compared to many MMORPG. The game itself isn't very popular. Updates comes pretty slowly even though there are events frequently.
Giving the game a bunch of nerf isn't going to help Mabinogi. Just look at Mabination. The forum activity has gone done a lot over the years. Many people quitting the game, or not playing as much as they used to.