WARNING: The following contains spoilers from Chapter #72 of the Dragon Ball Super manga, written by Akira Toriyama and illustrated by Toyotarou.
Goku's Spirit Bomb is one of the most famous attacks in Dragon Ball lore. However, it is possibly the most inconvenient attack to actually use. Gathering energy from life forms both near and far, charging this massive ki blast requires the user to leave themselves completely vulnerable.
Now that the Dragon Ball Super manga has confirmed that Ultra Instinct is a state of mind -- not a transformation -- could the mighty Kakarot possibly combine these two techniques to create the ultimate attack?
Goku must be in his base form to gather energy, lest the various Super Saiyan forms' maliciousness cause that energy to turn on him. In Chapter #72 of the Dragon Ball Super manga, Goku is able to activate Ultra Instinct while in his Super Saiyan God form. This combination opens the door for him to use the Spirit Bomb in the Ultra Instinct state, now that it's confirmed that the power of the Angels is not a transformation.
While powerful, the Spirit Bomb definitely has room for improvement. It failed to defeat Vegeta or Frieza, and Jiren was able to throw it back with little effort. Even Kid Buu would have successfully tossed it back at Goku had they not cheated and used the Namekian Dragon Balls to restore the exhausted Saiyan's power back to normal.
Goku's vulnerability is the Spirit Bomb's biggest flaw, as Piccolo and Vegeta can attest to after both served as meat shields while their Saiyan comrade charged his attack. If he were to charge the Spirit Bomb in the Ultra Instinct state, he would no longer be so defenseless. In the anime, Goku is able to charge a Kamehameha Wave while dodging with Ultra Instinct, so the pieces for this hypothetical new move are already there.
As Goku's last resort, the Spirit Bomb is mostly seen in situations that are too dire to charge the blast for very long. Only Jiren gave him as much time as he wanted, but there were only eight people to gather energy from after Vegeta refused to help. It's possible that the Spirit Bomb has such a poor track record because it has never been used to its full potential. It's unknown how powerful the attack would be if Goku could gather energy for as long as he wanted.
The Dragon Ball protagonist is already so overpowered that he may not be a suitable main character for much longer. However, this hypothetical technique likely wouldn't push him over that precipice just yet. As a last resort attack should be, the Spirit Bomb is an all-or-nothing move. The Earth's greatest hero has a (very bad) habit of letting villains go, rather than terminating the threat. It's unlikely that he would use the Spirit Bomb very often, Ultra Instinct or not.
Fortunately for his enemies, Goku remains a rookie in terms of Ultra Instinct. Still struggling to simply dodge attacks -- the technique's bare minimum -- the battle-hungry fighter is a long way from mastering his new skill. However, adding an Ultra Instinct Spirit Bomb to his repertoire would certainly be a game-changer in Dragon Ball Super.