I think a new type should only be seriously considered if AT LEAST 1 line for every generation could have it (either added on or replacing its current type). After that, type matchups and new moves and so on can be thought of next

For instance, the fan-favorite Sound type;
>Gen1 = Wigglytuff (Sound/Fairy)
>Gen2 = Misdreavus (Ghost/Sound)
>Gen3 = Exploud (Sound)
>Gen4 = Kricketune (Bug/Sound)
>Gen5 = Meloetta (Sound/Psychic-Fighting)
>Gen6 = Noivern (Dragon/Sound) or if that spergs you, Pyroar Fire/Sound
>Gen7 = Kommo-o (Dragon/Sound)