I think that dungeon idea is quite nice, it require a lot of work but profit would be also big. You have my axe 
Recently i wrote about some ideas, new content that could also be added at some point in game (after reaching certain level or iou score - it was here https://iourpg.com/forum/showthread.php?tid=2291), today my 5 cents is how about adding to game personal building/castle/HQ? My suggestion is giving player house/base that he would upgrade (after a couple of levels that building would change a bit look for more majestic/badass) and it would give player some small passive bonuses. Upgrading would require a mix of current resources or a new one.
Second idea connected with previous one is to give player a couple of buildings, each one specialized in different bonus and player would decide what building (and bonus that building provide) to "activate" on fiting for him time.
(example) Barracks/Training Hall would give damage, spending 40T gold (or some new currency added to game) would activate +20% auto dmg for next 2h
Mode unlockable at 400 or 500lvl (but i think the earlier is better).
Recently i wrote about some ideas, new content that could also be added at some point in game (after reaching certain level or iou score - it was here https://iourpg.com/forum/showthread.php?tid=2291), today my 5 cents is how about adding to game personal building/castle/HQ? My suggestion is giving player house/base that he would upgrade (after a couple of levels that building would change a bit look for more majestic/badass) and it would give player some small passive bonuses. Upgrading would require a mix of current resources or a new one.
Second idea connected with previous one is to give player a couple of buildings, each one specialized in different bonus and player would decide what building (and bonus that building provide) to "activate" on fiting for him time.
(example) Barracks/Training Hall would give damage, spending 40T gold (or some new currency added to game) would activate +20% auto dmg for next 2h
Mode unlockable at 400 or 500lvl (but i think the earlier is better).