User blog comment:DBard/Thoughts on True Longinus and God's Dying Will/@comment-1359322-20130725004228/@comment-1359322-20130912060156

Nah even if Issei gotten his hands on the Longinus, he won't be able to use them since it isn't his, same goes to Indra, he only confiscated them, he can't use it. To be able to use Sacred Gear or Longinus, they need to go through a ritual like the one Raynare did to make the Sacred Gear or Longinus his or her own but will kill the original users. Well if there's anything it's more to this:

“It seems like Indra punished all of the Longinus possessor from the Hero-faction. That includes Cao Cao, Georg, and Leonardo. But according to Indra, he confiscated the spear, and sent those three to Hades.”

Though, he didn’t hand us the holy-spear. That bastard Śakra is probably in possession of Dimension Lost and Annihilation Maker.

It will make him look like the one who finished off the Hero-faction. Even though he lent his strength to them. He used them to the very end, and he attained a reason for being in possession of the Longinus.

[It can’t be helped for Śakra who punished the Hero-faction to be in possession of the Longinus for a temporary time.]

(Quoted from Volume 14 Romania)

As for the True Longinus choosing Issei's dreams is probably more to the effects of the Truth Idea.

Truth Idea's effects depend on God's will in the Holy Spear and sucks the ambition of the holder of this spear, and by responding to how strong the opponent is. So it probably deemed Cao Cao's ambition as "unworthy" and chose Issei's dream instead. Remember Issei's dream isn't all on being Harem King, there's also the peace and being hero to the kids of the Underworld. The will probably thought that Issei's dream of a hero is better than Cao Cao's ambition of a hero.