Any niche they want. The problem is that wrestling niches are so miniature, it doesn't make any business sense to cater to them. We have people who like Hardcore, High-Flying, "Pure" Wrestling, Cursing&Boobs and then there's casual which is very little of all of those other niches and more "entertainment" (I hate that word now).
If you cater to the Hardcore audience, yeah you please all 34 of them, but then people bitch about:
*You have no stories
*You have no characters
*You only focus on "xtreme" which gets boring
*Likely low production values
*Too dangerous
If you cater to the High-Flying audience, you sure as hell will please them but ...
*You have no stories
*You have no characters
*All matches are spot fests
*Production values are crappy
*Too dangerous
If you cater to the Pure Wrestling fans, yeah they'll bust a nut but ...
*You have no stories
*You have no characters
*It takes seven working days to complete a match
*What are production values?
*Too snobby
*ROH already did it stop ripping them off TNg@y
If you cater to the Cursing&Boobs fans, there shall be *********ion taking place but
*Your stories are offensive
*Your characters are offensive
*Everything is done for shock value which sucks
*WWF already did it and did it better
*Get over the Attitude Era
*Not enough good wrestling
So in the end, yeah you COULD cater to any of those. But do any of them have a following big enough to invest so much time, money and effort in building a product for those people? It's a bit difficult to provide market research on wrestling audiences (I assume). In the end, the casual fans remain the largest segment and that's why everyone is catering to them.
The question is, IS this all true? What if there is a high demand for hardcore, or for high flying, or for "pure"? I think the answer lies in TNA having to combine all niches while providing nice elements of entertainment such as stories, characters and maybe fine production values if the budget allows it. That way you do all of the things WWE is deathly afraid of doing (associating with a niche), and you take literally the only good things about "entertainment" - focus on stories and characters. Then you might have something.
One thing that is proven in wrestling is that focusing on one single niche is a death wish. ECW did it and while their niche was noisy, ECW died quickly. ROH is doing it and they're not doing too well. Indies have been doing it for ages and maybe that's why so many of them remain indies. Mixing and matching is the best option. Wrestling fans never like just one thing. I think we need a bit of everything, and make it good. Point is we like wrestling, and in 2015 wrestling can include all of those things above in fine amounts. Then you have diversity which is really attractive to viewers.