Here's some more information I could dig up, mostly about the Item World.
Unlike before level spheres need to be broken instead of lifted in order to get their bonus, then there are random rabbit enemies that give a random boost to the item when smacked.
This person got their bonus gauge up to 6 and gained 3 extra levels for an item. Bonus 0 (default bonus) gives the level clear level and then there's the bonuses. They are at bonuses 3, 6 and 9 you get an additional item level increase
https://www.jp.playstation.com/blog/20150318_disgaea5_12.jpg
This means clearing 10 floors and maxing out the Bonus Gauge could very well reward like 40+ levels when you factor things like Level Spheres and Rabbit Monster bonuses.
Quite a few interesting things here
https://www.jp.playstation.com/blog/detail/282/20150318_disgaea5.html
Apparently there aren't pirates anymore but "Invaders"
Statisticians will cap at 900 instead of 300 now and they don't stack beyond that. This means you can fit 900 Statistician using only a single Innocent slot but only max out at 900% exp bonus for a single character. However if the Cheat Shop 1000% Exp is multiplicative like in previous games then this isn't an issue at all. (900 x 1000% = 9000%)
Rather than duping with the Puppy Paw Stick, there is a new NPC in Item World that will randomly appear that duplicates the item you are currently in.
Random Encounters - An Angel can appear in Item World that will spawn a neutral NPC. When you kill this NPC they give you items. The Angel can also spawn a Super Boss. (Darkdeath Evilman for example)
You get whatever you earned from the bonus list, even if you skip the floor. So doing tricks to max out the bonus gauge such as using Geo Panels or use a high leveled Thief to Create a Box. Then have some weak characters mass combo it to max out the bonus gauge and then just skip floor to reap the rewards.
So far this sounds to be in terms of mechanics the best Disgaea game to date.