I also wanna point out, Loki joining the crew can indeally counter Kuzan joining Blackbeard.
Its not even about him fighting SanJuan Wolf directly, even though esthetically they'll be visually matched up on panels because of symmetry.
The Kuzan vs Loki pairing cancel eachother out in theory because both have a similar skillset, potentially bringing their fight down to haki, which Kuzan has proved he can keep up with the likes of Garp and Akainu with, and even Blackbeard opted not to fight him despite having half his crew on stand by.
If Loki doesn't join it's going to be because of a size gag or suddenly the citizens of Elbaph and even Harujidin want him to be king despite that never being apart of his own dreams.
Loki's personal goals are tied to Rocks and Galleila.