When you say "Champion that would fit more with the theme", do you mean Champions who already have ties to the Celestial Realm? Champions who have some sort of cosmic influence in their character? Etc. etc.?
'Cause the Dark Star skinline hasn't ever really been restricted to Champions with any particular cosmic affinity. Like, let's look at the skinline real quick;
Dark Star Thresh - A sadistic, spectral jailor/tormentor
Dark Star Varus - A Darkin archer hell-bent on vengeance
Dark Star Kha'Zix - A Voidspawn who hunts down great prey to evolve
Dark Star Orianna - A girl made almost entirely of Hextech
Dark Star Jarvan IV - An audacious human prince with a great strategic mind
Dark Star Cho'Gath - A gluttonous, devouring Voidspawn
Dark Star Karma - The umpteenth incarnation of an Ionian spiritual leader
Dark Star Shaco - A corrupted marionette with a penchant for murderous "games"
Barely any of those Champions have anything to do with cosmic stuff, and I say "barely" because I can hardly count Kha'Zix and Cho'Gath, since their origin (the Void) is a plane of stagnant existence, and to that end isn't really cosmic in theme, and even if it were, both of those Champions seem only to serve as the Void's hunting/killing creations.
Of course, I'm describing the Champions in really, really short blurbs (and doing Orianna, most out of all of them, a serious injustice), but the gist of it is still there; Dark Star has no precedent for favoring Champions with cosmic/celestial thematics.
Personally, I think the best Dark Star skins come from Champions who are innately inhuman; Thresh, Orianna, and Shaco probably the best-fitting, but monsters like Cho'Gath and Kha'Zix fit well too, though Varus, Jarvan IV, and Karma still can (and do) work for the skinline.
And, sure, Bard is innately inhuman, but, that and his being tied to a cosmic thematic doesn't mean he qualifies for the Dark Star line better than any other Champion.
