Thanks, I appreciate constructive feedback like this!
And I think it's a great idea... I think that once Natsu manages to gather the main members of Fairy Tail, he could create a main character who joins them...
What you do need to keep in mind is that even though we control Natsu now, in many missions we'll control different characters, and we might see different sexual scenes between them, for example...
I wouldn't see a sexual scene between Natsu and Juvia as feasible, but one between Gray and Juvia would be.
But, introducing a main character who establishes a relationship from scratch would be a good idea to have a character who can forge bonds with everyone from the beginning...
Sorry for this long-winded explanation, but your comment really got me thinking, haha.