It's not a terrible idea. Writing a predefined character allows BioWare to write a better and more coherent story with a more emotionally involved protagonist who actually has some depth. There is a long list of videogame characters that are memorable and really left a mark on me, but boring blank-slate self-insert characters like Shepard aren't on that list. Geralt however, who isn't a boring blank-slate self-insert character is definitely on that list.
The only issue is that the game becomes less of a RPG the more defined the protagonist is. Geralt works from what I've heard (I need to actually sit down and make the time to play this series at some point), because they don't just use a set protagonist, they practically revel in their making of Geralt and literally examine ever facet of Geralt without any restrictions. It's not necessarily So what ends up happening is that you, the player, become invested in what Geralt would say/do because you want to see more about Geralt, and you want to hear more about how people react to the things Geralt says. It works, not because Geralt is a set character, but because they continue to utilize Geralt's background to establish his earlier dialogue options, and later on, never stop expanded on his base personality that was set by his background earlier.
I'd believe what you said if Bioware ever showed that they were capable of handling things with defined protagonist. But we've seen what they do with set characters-- Revan, Hawke, and Sheperd. In Revan's case, Obsidian is the one that made Revan a more interesting character-- and that's more defined by Revan's feats than his/her (going off of the 2 KOTOR games ideas) than actual personality. Hawke has some funny lines, but there still isn't a lot of depth to Hawke and that's even with the triad-semi-locked personality Hawke had. Even if you were tough, sarcastic, or diplomatic, it still extended from the same vein of personality. Finally, we have Sheperd, who was a complete brick throughout the series. Sheperd was never smart, Sheperd was never interesting, and Sheperd being the most defined protagonist out of all of the characters never had any impact. And I'm not even talking about Sheperd not hurling some biotics in cutscenes if Sheperd was a biotic (hell, it's rather common for biotics to NOT USE THEIR BIOTICS during cutscenes unless they are supposed to do something *cool*), I'm talking about Sheperd not even referencing the fact that he/she was human. It really stopped mattering after Mass Effect 1. Why? Because even in Mass Effect 2 when you were working with Cerberus, even Cerberus was willing to work with non-humans. And not just one or two either, they were willing to abandon their HUMANS ONLY mentality for this part of the story. By 3, it really doesn't matter. Take back earth? Well considering what you have to do to take back earth, I'm not seeing how being human is a requirement.
Set backgrounds are cool when the writer actually allows them to DO SOMETHING, which they have not. Even with Hawke and the legacy DLC, it really doesn't do much in the sense of Hawke being more interesting here because 1) Hawke can possibly not learn about his/her heritage, 2) Even if Hawke does learn about his/her heritage, so what? A person is defined by their choices and actions, not by their birth. And that's one thing that I actually liked about the different races etc not focusing too much on what background you were. Because people don't act like that. If you were a farmhand and managed to get into Harvard, people aren't going to drone on about you being a farmhand, just the same if you came from a prestigious line of people that all went to Harvard, the most I'd expect someone to say is "oh, your family seems to always go here."
All and all, I only want human only if they absolutely go balls to the wall and every single aspect of your character is specifically defined to be distinctly human. otherwise there's no point and they should just expand on making you other races.