Speaking personally... I intend on making sure that if in fact I decide to use the "ally" concept at all (which is not guaranteed by any stretch at this point), that whomever gets selected will have a MUCH more important part to play than just granting Inspiration. That's a purely mechanical benefit and has no story implications... thus guaranteeing for me a complete waste of time. For me, this entire campaign is about story, and any mechanics are a DISTANT second, third fourth, etc.
Thus... if by chance Arabelle was selected by the cards as the ally, I would create a much more important story about her and why she is imperative to being with the group when they confront Strahd. Off the top of my head... the very first thing I think of would be to make Ireena Koylana a 'bait and switch'. Set the story up such that the party AND Strahd think Ireena is the reincarnation of Tatiana (like the book dictates), but have it be revealed down the road that in fact what everyone thought was true, was wrong. And in fact Arabelle is the true reincarnation of Tatiana, and allow that fact to impact everybody else. What does Strahd do when he figures out that the love of his life is currently a 7 year old girl? Has he ever faced this situation previously when the discovery of a Tatiana reincarnation was this young (and not young adult?) How does he react? What does he or doesn't he do if she's in the party's presence? Can he bite and drink the blood of a little girl? Could he not do it for some reason? If he can't, does that grant the party the advantage they need? How does Arabelle/Tatiana impact the story?
Or another track: Strahd is looking for an heir to take over his realm, but maybe finding an heir isn't just enough? What exactly is Arabelle's destiny? Maybe perhaps she's destined to be the most powerful Vistana ever? Or destined to assume Madam Eva's role? Perhaps Strahd's time in Barovia is directly connected to his half-sister Madam Eva's time in Barovia... and it's not just enough for him to find an heir to assume control of the land, but perhaps Madam Eva needs an heir to her position as well. And only when a true replacement for both Strahd and Eva are in place can the both of them move on. Thus, perhaps the party needs to help Arabelle come into power as well as finding someone to assume Strahd's position, at which point Strahd and Eva move on and the demiplane becomes "cleansed"?
There's a lot of different ways to go about it if you're willing to find and create new endgames that are more than just "Go to the castle with the ally, hit Strahd with the Sunsword until he dies, everyone goes home."
