This has brought up two primary questions:
1. Will we reach that desperate point of having Lighting dressed as Agent 47?
2. Why is there so much of this?
While no official statement has been made by SquareEnix or the developers behind the game, here's a few possibilities why. None of which are good.
The first and most obvious of these is money. If you have seen many releases in the past year, a staggering number of major titles have invented reasons for day one or exclusive DLC. If it's an FPS it might be be a new gun, if it's an RPG it might be a figurine and so on and so forth. The worst of these tend to have exclusives appearing with specific retail chains, looking at you Aliens: Colonial Marines, which are done to create hype but primarily to encourage people to part with their money quicker. It's less a means to reward people and more a way to ensure that the developers are getting as much cash as humanly possible, something only more damning when you consider a lot of Lightning Returns stuff is DLC available from release.
In an apt comparison between Lightning Returns and the aforementioned Aliens trainwreck, the DLC seems to be trying desperately to invoke nostalgia for the older games. Between the outfits of Cloud, Aerith and Yuna there is already a big emphasis upon trying to draw back fans of the series' biggest hits. Those most likely to part with their cash thanks to seeing something they have very fond memories of, just like Aliens did with its multiplayer skinpack of the film's characters.
This would not be the first time SquareEnix has attempted to farm cash from their fans via having characters (or in this case their costumes) involved. Many know of the insane pricing and cost scheme found within Final Fantasy All The Bravest. A title heavily criticised for robbing the players of all meaningful control (insert obvious XIII joke here) and prevented any major progress without going to the cash shop. Many more know of certain costumes which showed up in Final Fantasy XIII-2, such as the horrifically out of place Mass Effect armour which similarly came as DLC. While such tributes and easter eggs might have been present in previous titles, they were traditionally background additions and did not require you to fork over more cash. This change in policy in recent years either suggests greed on the publisher's part, or something else.
Despite their acclaim, SquareEnix deemed many successful titles such as Tomb Raider, Hitman Absolution and Sleeping Dogs to be financial failures. This has been attributed to them not meeting a number of astronomically high goals with Tomb Raider in particular required to have sold seven million copies in order to make a profit. Between this and other news such as the publisher's five-point-seven billion yen loss in 2013, it seems as if the publisher is banking on this being a major success. As a result, these efforts to pile on pre-order packages and early DLC could be seen as an attempt to make up for their previous financial losses. Something desperate to be sure given the number of major IPs under their control.
Just for the sake of argument however, let's ignore the financial side of things and consider what the DLC means for the game itself. This is the third time in which the series has visited the world of Cocoon/Grand Pulse and it has been given far longer to build up a major mythos, history and well rounded characters than any other title. The only possible exception to this is VII, which found its success long before any spin-offs, and X with X-2 re-using many elements from the previous title. Despite this they are not resorting to bring back memories of those past games with this DLC, but instead try to focus upon entirely different titles. This suggests one thing: A lack of faith that the world the developers created, the one the series has been stuck on for three games, is enough to bring in fans.
This is not to say that there are not people who enjoy XIII or even love the characters, but that SquareEnix knows of the general apathy towards their games. They know of the heavy criticisms behind XIII, the lack of development within the world beyond spending hours reading through the codex, and the lack of interest in the heroes. As a result they are resorting to try and get fans back by having the protagonist dress as more widely liked icons.
Naturally this is only a theory, but it seems unlikely that SquareEnix would not notice the general dislike for the games nor the heavy criticisms which have been leveled at the world in which these games were set. Were they more confident in their title, they would likely not be reaching for other markets and instead looking to draw back fans by reflecting upon those past games. Rather than seeing Lara Croft's attire, we would likely be seeing Lightning's original outfit as DLC or something to bring back elements present in the first game.
However, that is only one other potential reason for this. This could well be another desperate attempt to get audiences to love Lightning. The character herself has been criticised as bland at best and unlikable at worst, yet her role has been boosted with every preceding title. This has reached the point where she has gone as one resistance fighter among a group, to a divine figure with the entire second game devoted to characters trying to help her, and now with the character on her own as a redeeming goddess. The writers have opted not only to shunting every other character to become unplayable side figures but turning her into the single most important being in existence. Atop of this Lightning has turned up in other games, most notably XIV with a quest and reward which aggrandised her, and heavy promotional materials are focused purely upon her. Even the Final Fantasy title itself has been moved around to put her name first, ahead of the more iconic franchise name.
The forces behind the Final Fantasy games have admitted they developed an "obsessive fixation" upon Lightning, and some display signs of having fallen in lover with their own creation. Just look at some of the comments quoted here. The character has been repeatedly promoted, sometimes to the detriment of past heroes. She was at one point cited to be the first female protagonist, earning the eternal ire of everyone who played Final Fantasy VI, and even described as a "female Cloud" at one point. As a result, this seems like one last desperate attempt by some figures with a say in how the game develops to try and make people love Lightning as much as they do. An effort to latch onto the popularity of heroes with fanbases by having her effectively cosplay as them and hope that this somehow translates some of that popularity onto her.
This is just a series of theories about the game based upon industry trends, information from interviews and similar details. Please only quote this as such. Just consider it to be an effort to make sense of the recent attempts to push DLC and the shifts in how the series has approached its audiences. Short of the developers themselves coming out and spilling the beans, it's really the best anyone can do.