After you complete the "Main Quests", you will want to complete as many quests as possible. They offer TONS of cash, and you can find... *interesting* weapons.
Besides from that, once you complete to a certain point, go through the towns' dialogues and you'll eventually open a new place to explore. It will show up on your map, and have monster-girls fight you, before you come to a Succubus tower, with 2 new girls, then the boss succubus.
It's extremely difficult to win. I was stuck with 3 healer bosses that either completely healed all the damage I deal, or getting spammed by lewds.
I realized that they randomly generate in the tower, so I just left, came back, then just had to fight some easy-peasy girls instead.
The only two missions I can't complete is the mission that requires I do 80K damage in one strike...
And some weird quest. Since I can't understand the dialogue, I have no idea what to do. It's just two mages, point blank, and if I kill them both, it immediately fails me, wether or not I kill them simultaneously. there's no timer, and no health bar, so they're not bosses.
Besides those 2, I think that's all I have left to complete. I think I'm either done, or missing 2 or so in the library.