The main reason why I'm more attached to Stella's character than Luna's is because of this one scene:
https://www.youtube.com/watch?v=4n_x5k9h1Hg
Her characterization was just so much better in that one scene than anything we've seen from Luna so far. Stella's story may have been rewritten numerous times (first that scene was were they first met, then they were childhood friends, and then she became Luna), but that shouldn't have been a reason for her characterization (nor her design, for that matter) to get downgraded this much. Of course we haven't seen any conversations between Luna and Noctis (in present day at least) yet, so it might turn out very similar. But that just means the marketing team and Tabata haven't properly been highlighting why getting rid of Stella was a good thing yet.
I feel like the reason why her story was scrapped was because it was too closely tied to "the light of expiring souls" and the powers received from them. Note that this light was a huge theme in Versus, but did not occur once in any FFXV trailer (except the first, which also still had Stella and was still Versus in anything but name). You could also actually see this light when Noctis defeated an enemy in Versus (look closely at 3:53 and beyond in
this trailer), but not in FFXV. I know Noctis still had his near-death experience, but I think the light is scrapped.
This is pure speculation, but in the scene were Stella and Noctis were drawing their weapons against each other, there were those huge magic signs behind them, and I think those were results of the powers received from seeing the light of expiring souls. You'd think that if those powers were still in the game, we would have heard about it by now, since they'd be a pretty huge gameplay element. But we've only heard about ring magic and the "normal" type of magic. We haven't seen ring magic yet, but I don't think that's it.
So to me it sounds like Tabata or his team decided to earn some goodwill by throwing Fabula Nova Crystallis out, and with it the light of expiring souls. Then they saw Stella's story didn't make sense anymore and threw that out as well. I don't know, maybe this is pretty harsh but if it didn't go down like that then they just should have shown that some of these concepts remained in the game.
On a side note, that scene also shows visual direction and dialogue far more powerful than anything we've seen from FFXV. The marketing team should really just show something of this quality in their trailers - if there is anything left after Nomura's departure.