Simply 2 characters that fused once in series (Ofc You can add alternate Fusions like UE Vegeta + MUI Goku , Orange Piccolo + Beast Gohan Etc...)Must Fuse in game too which makes fusion comes with full health bar(If Both characters have red bar Fusion will come with Green full bar),Example:
Super Vegeta + SS Goku = Z Gogeta (Both Potara and Dance must have a time limit Depends on the tech,plus theres Supposed to Be a mechanic, ıf Both characters fused in SS The Fusion will come up with SSJ, If Both characters fused in base form or SSB form The fusion Must come in base form or SSB Form As I said) The Fusion must Being able to used ıf Both characters is 3 levels, Im not telling that fusion ability Must added to all characters, ofc theres limited characters and you can add non Canon Fusions too+There Must be mechanic Which decides perfection of the fusion dance (Potara doesnt count and ıf both characters that going to Fuse and they capable of doing both potara and fusion dance then we Must give a choice between them) Simply Fusion dance mechanic is:

The Answer: 2039 (Answer Must disappear in 1.5 Seconds, Which Simply tests your Memory)

The Numbers: 0,1,2,3,4,5,6,7,8,9 (This numbers will appear for 4 times and you must enter the Numbers Same with the answer given you, If you do this Wrong for example instead of gogeta you will get veku and wait for the time limit end and get characters to level 3 again)