Baity hacks >:
If you're interested in post-game content, the first part is this
Ver.2 bosses on 4F(1), 5F(3?), 9F(Bloodstained), 10F(1), 12F(2), 18F(1), 19F(4), and 20F(5 and a bloodstained). Then after beating all 4 bloodstained bosses you unlock the final one on 1F.
Also, Boss Rush right below relay point at 20F. You get the final item for beating that.
You get a star for beating the game, collecting all 32 non-plus characters, all 100 items, all bloodstained bosses beaten, and all ver.2 bosses beaten. Getting stars lets you pass through more of the new teleporters, and with all 5 you can fight Final Boss Ver.2 (Which gets you 2 new characters for winning)