I want to be able to play as a dedicated diplomat or a disinterested wanderer if I please.
There is already a bit of Neutral roleplay. You may not give a damn about Sazza, Arabella or Liam. You're just trying to find a healer and head back home. In that case, you're not going to go out of your way to help other people (my definition of Good), but you're not causing any suffering to others for your personal gain (my definition of Evil). You're just a (possibly hardened-hearted) passer-by. A disinterested wanderer, I suppose.
To me, if you save the tieflings because you sympathise with their plight and want to help them, you're Good. Maybe you're very brave and not afraid of taking on armies of goblins and you make sure to kill all the goblins, so that the region will be ultra-safe for the tieflings' passage (no new leader emerging and rallying the goblins). But you could be more Neutral and end up saving the tieflings merely because Halsin wouldn't be available to heal you until the goblin leadership was dead (he himself has his own reason for wanting that, and it's not to help the tieflings).
The difficulty with Neutral, is that it's not so much a clear ideal and morality. It's often what is neither Good nor Evil, and presumably in the middle (as Good and Evil are often thought of as being on a one-dimensional spectrum, which certainly is restrictive). As a result, Neutral could be narrower, or much broader, than Evil or Good, depending on how far you see these areas expanding toward the middle.
I guess Larian should start by ensuring that they have equally good and motivated paths for both Good and Evil. Then find how much room there is in-between the two paths. Ideally, Neutral shouldn't end up being the same result as Good, but for slightly different motivations. That being said, I'd already be happy if there's a good Evil path and a good Good path. Having an interesting Neutral path sounds like the icing on the cake.
Note: in fact, probably, they shouldn't start by thinking of writing a Good path and an Evil path, but they should start think in terms of "how could things naturally unfold, what could various characters do, seek, and how would that affect the end situation".