With the list of Thor's living Asgardian allies the shortest it's ever been, director Taika Waititi could be looking to round out the cast. If Marvel brings back Lady Sif, how exactly would they go about it?
To determine how Lady Sif can return in Thor: Love and Thunder, it is important to determine Lady Sif had died off-screen following Thanos' Snap at the climax of Avengers: Infinity War.
Although fans didn't get to see it on-screen, Lady Sif would have been brought back to life following the successful endeavors of the Avengers to bring everybody back from the dead in Loki Disney+ show, but the characters had precious little interaction in the two Thor chapters they both appeared in (aside from a mutual distaste for the other), meaning the most logical place for her return is in Thor: Love and Thunder.
Lady Sif Can Setup Jane Foster's Arrival As The New Thor
The last time audiences saw Thor, he was departing for the final frontier with the Nidavellir (set up in all three Thor movies) to ask Eitri, or even the warrior's afterlife - Valhalla - to ask Heimdall. This can end up with her being told where 'Thor' is, only to find Jane Foster in the armor. Then, Lady Sif could continue on to find the original Thor and inform him of the existence of the new one. This would pull both Lady Sif and Thor back to Earth, either to New Asgard or wherever it is that the new Thor is operating.
Lady Sif & Thor Will Return To New Asgard Together
Valkyrie (Tessa Thompson) is now the leader of the remaining Asgardians residing in New Asgard on Earth following the events of Avengers: Endgame, and Thompson is confirmed to be returning in Thor: Love and Thunder. Thor himself could make a return to New Asgard, if only to interact with Valkyrie. It would make perfect sense for Lady Sif to return there as well.
After being absent from the destruction of Asgard, and having been returned from the Snap, the character would look to be with her people in their new home so as not to be alone; perhaps even from a sense of guilt stemming from not being present when they were under attack by Hela and later, Thanos. Combined with the previously-mentioned potential link to discovering the new Thor, it's plausible that Lady Sif and either of the Thors will return to New Asgard together.
Lady Sif Could Become Valkyrie's Queen In Thor 4
It's up for debate as to Valkyrie has been announced as being LGBTQ by Kevin Feige, it's even possible that Lady Sif returns to be a love interest for her (after all, the film explicitly has the word 'Love' in the title). Either that, or Thor: Love and Thunder has Thor coming face-to-face with three of his past romantic interests - Jane Foster, Valkyrie, and Lady Sif - for the purpose of dramatic irony within the film's plot.
The love quartet is made even more engaging if Thor returns to New Asgard alongside a new Thor (Jane Foster) and Lady Sif is sitting on the throne. As Lady Sif was previously rejected by Thor (albeit, an unspoken rejection in Thor: The Dark World), to flip the power dynamic on him and have her return as Queen of Asgard would make for an exciting development in Thor: Love and Thunder.
