I think Young Indy is there to give Deb a means of escaping the country once Dexter's secret is exposed and people start putting together the pieces about his victims and what happened to La Guerta. He's independently wealthy, smart, knows police tactics, and has few major ties keeping him in the country now that he's distanced his sister from her creep boyfriend. They might even take Harrison with them if Dexter knows he's going to be caught or killed well in advance.
As far as Psycho Lad goes, if they have him try and kill off Dexter in the Sith master & apprentice style, or have him implied to be the new Dexter carrying on the code once Dexter is in prison or dead at the end of the series, then I won't mind the introduction. If he's just another kill for Dexter though, without any further payoff, that would seem wasteful, especially with introducing Dr. Vogel into the mix and having her teach him the Code along with Dex.
I do think there is some potential for conflict between Dexter and Psycho Lad given the profile of women he's killed so far. While up to now they've all been women his dad has cheated on his mother with, they've also all been Hispanic domestics. So maybe that becomes his "type." And given that Quinn is the one following him and Dexter is the one training him, that gives two different ways for him to possibly become obsessed with Jamie Batista as a target, which would place her and possibly Harrison at risk and give Dexter ample reason to end him.
Not sure how Hannah figures into all this yet, except maybe as the way to expose Dexter to the police, or as one final kill that Deb has to make which convinces her once and for all that she needs to get away from Dex and Miami and leave the country.
