You are correct in suspecting that there is no real subject. The construction “Il pourrait arriver [quelque chose]” is similar to “il pleut”, “il y a”, in that the subject “il” exists only to satisfy the grammatical need for a subject.

This dummy placeholder is always “il”; never feminine, never plural.

“Il pourrait [lui] arriver” can be followed by a number of objects, such as “malheur”, “une bricole” (meaning something minor but annoying), “quelque chose {de bien, de désagréable, …}”, and even “une bonne chose” but “joie” and even “bonheur” seem very strange. I think the construction is very set and does not admit much variation. Any variation would depend on “quelque chose”: “quelque chose de …” or “quelque chose qui …”