I both do and don't wish that they'd written Tom Paris' character as Nick Locarno, as they seem to have considered at one point.
I am very interested in guilt and shame as themes, and I think Nick in Voyager would've been a very interesting exploration. Guilt and shame is what Tom's story begins as, so you can see the outline of Nick's character there... but I do feel like it would be a different story.
We have Tom explain the event that got him removed from Starfleet only once, if I recall correctly. And when he explains it, he says that the deaths of the other people in his mission were due to "pilot error." A Starfleet pilot making some "error" during a mission (Tom's background) is a much less disconcerting image than an upperclassmen intentionally rallying younger students into a dangerous stunt (Nick's background).
Both characters try to cover up their culpability, but Tom voluntarily confesses afterwards. He chose to speak up even after he "got away" with his cover up, which shows us that he wasn't willing to lie about those deaths for the sake of his career. That's not something we get from Locarno. Nick pressures younger students -- whom he's responsible for leading -- into lying, and he only owns up after someone else comes forward... his actions are more difficult to swallow than Tom's.
All that to say, Nick's backstory is just more disturbing, imo. Not only were his actions more unethical, but he was also a young adult when he did them. Knowing that all of this happened among children makes it that much more painful. And none of this is to mention that the Voyager audience would've actually seen this character do the bad thing they're known for.
Honestly, I think Voyager shied away from reminding the audience of what Tom did. He only recounts it once, if I recall correctly, and a lot of his character beats about guilt are made to orbit around a vague sense of failing to be what people expected of him. This is no less real a story, and it is a good story imo. I don't think it's necessary that Voyager make everything about a particular event for the guilt theme to be worthwhile. All that to say, changing Tom out for Nick wouldn't have been a small change. It would've made his story quite different, and -- inevitably, I think -- more challenging to watch.
If we'd had Nick, we would've had a character with a whole TNG episode dedicated to giving us intimate knowledge of his deeply unethical decisions. The advantage is, it would've made the shame he struggles with in Voyager more visceral. The audience would not easily forget how immoral his actions were, nor how committed/confident he was in doing them. Any moment where he expresses guilt would hit much harder, because we'd actually be familiar with the events he's feeling guilty about. Remembering how Nick's actions affected Wesley -- not to mention, being able to put a name to the person whose death he's responsible for -- would've made it easier for the audience to feel the anger and resentment towards him that other characters in Voyager do. And honestly, I think that would make the theme of shame more real.
Shame is such a beast to live with. When you enter a room, you feel (accurately or not) that the people around you don't think you deserve to be there. It can be physically painful to just be seen when you feel that way. We could easily imagine feeling that way, if we were being asked to put ourselves in Nick's shoes. And that would bring us to the question, what is Locarno left to do, knowing that he did something inexcusable? Shame complicates everything. If you come to believe that you don't deserve to be among others at all, how are you mean to live? How are you meant to engage in the world and be someone, let alone become someone? That's an extreme way of phrasing things, sure, but I think we see moments of Tom feeling this way in Voyager.
In the Threshold episode, Tom tells Janeway that he believes she and the crew are lying when they say good things about him (aka, when they indicate that he deserves to be there). During the plotline where Tom leaves Voyager as part of a secret plan, he quite convincingly pretends that he doesn't believe he fits in on Voyager anymore-- it's a performance, yes, but I think it's implied that Tom knows what to say because he's felt this way before. And then there's the episode where Harry enters an alternate universe where he never boarded Voyager, and he meets a version of Tom that says he hasn't cared about his own future in a long time.
What Tom does in the show is work through those feelings of shame in order to collaborate with others on Voyager, and I am intrigued by the idea of Nick going through the same character arc. I think it would necessitate some darker scenes than the ones we get with Tom. It would have to involve him developing a different moral compass than the one he operates with in The First Duty. I think it would've fit into Voyager's themes, though, and Star Trek's general theme of hope and human growth.
All that said, it would be emotionally excruciating to watch. And that's why I'm both disappointed and glad that they didn't write Nick in Tom's role. Would I be interested in that story? Yes. Would it be easy to watch? No. Would Voyager have been able to pull it off? ...I don't know.
Anyway. If I ever seem overly enthusiastic about Nick Locarno, this is why.
