I'm not saying the tile shouldn't happen, but I think It's not at all important and it's most certainly not an omission.
But the reasons for me saying this are kinda different. Isabella knows Zevran (he killed her husband and she slept with him 'cause of that) and he had too many chances to die to be ever featured in a game ever again anyway. Isabella will always know who Leliana is, should the two of them meet again (they meet in DA2). The same goes for Alistar. Not to mention that the comics most likely did happen no matter what and they would have far broader impact on Alistar - Isabella relationship then their meeting in DA:O. The only thing that isn't reflected in the Keep is if Isabella met Warden or not.
And this is just my personal opinion but I believe Bioware is trying to move away from Warden as far as possible, there are just too many variables (not to mention the lack of voice actor) for him/her to ever appear again in a physical form. The amount of people that would go "this is not what MY Warden would say here" is just too big for them to be able to deal with it. Also, he/she can die for gods sake, and died in canon. Also, at which point would in a conversation between Inquisitor and Isabella would her relationship with Warden ever came up? Do you expect an "Can you give me a list of all important people you have ever slept with?" question?
Then you have the comics, if they are anything to go by then the Isabella we meet, should we ever meet her, is going to be a very different person indeed.
In short, there is no need for such a tile, if you are worried about future meetings between Isabella and Zevran/Alistar/Leliana/Warden. IN game(s). But there are also a lot of tiles in the Keep that I'm sure are never going to matter so adding one more can't hurt anyone.
I obviously strongly disagree about the Warden reappearing in future games (or Zevran for that matter), in so far as I believe they should be present at least as much as Alistair in the grand scheme of things. Alistair has three times the oppurtunity to be dead as the Warden, right? Or at least 2/1. And Nathaniel has made returns as well, despite two oppurtunities to die. Then of course there are the canonically live characters who could be killed in previous games. Reappearing characters are important, even more when their appearance or the nature thereof is a result of player choices. So they don't get a pass from me on that.
It brings up an interesting question about outside media, though. If Zev was so important in Isabella's life, why no acknowledgment in DA:O? Because it was canonically introduced after the fact. And then we have this huge role for Alistair in outside media. But if he's dead, there's some alter-canon version of those events, but either way, most players will never see those events played out. So unless all the outside books etc. featuring present day characters are done in "Choice of" style game books (which I absolutely recommend btw; many of them are excellent) and like films etc, and those media directly effect characters that inhabit our worldstates, shouldn't it be possible for us to use the Keep to determine what version of those events took place in our continuities? That would be cool and helpful. Along with the continuing presence of our customizable NPCs, a.k.a. our former surviving protagonists.
edit: As for Warden and Isabella, going by DA2, she wears it like a badge of honor. As for how it would come up with the Inquisitor, I imagine it'd be something like: Isabella: "You're looking for the Hero of Ferelden? You know, I knew him, during the blight. The stories I could tell.. " Inquisitor: "Right. That's why I'm here. Apparently, you've told your 'stories' to half of Thedas. Anything more current I could persuade you to divulge?" And of course, there's the potential of future scenes, maybe even something like a DLC where they're party members. There's the new agent systems to consider that should take past ties into account. Maybe someday we'll get a choice someday of how things play out off camera to determine who with and how our characters show up at some point in the future. The list goes on.
The ability to kind of co-sculpt our stories over an extended life is really BioWare's great innovation. You never know where it can go, if it's fully embraced.