Since the nippon_ichi tag is applied very irregularly, it'd probably make sense for the games they developed to imply nippon_ichi. I believe this is all of them that are on danbooru (with disgaea already being implied by the individual games):
disgaea la_pucelle phantom_kingdom prinny_can_i_really_be_the_hero? phantom_brave rhapsody marl_kingdom soul_cradle hayarigami infiniteloop antiphona_no_seikahime
I don't know if we'd want to imply nippon_ichi from games they published in Japan or not, but if so:
criminal_girls classic_dungeon grimgrimoire tori_no_hoshi a_witch's_tale last_rebellion blue_roses
Also, on a related note, disgaea_infinite should imply disgaea.
Updated by Xabid