I dont think it would be cool since the whole doppelganger story is getting old fast with Katherine and Elena. Now Stefan and Silas. 

But, it would be cool if Tatia just showed up in TOs first (if she was a vampire) pretending to be Katherine. She shows up at TOs House and sees Hayley, they talk and before Hayley could ask who she was, Rebekah storms in and throws Tatia up against the wall and calls her, Katherine. Tatia says, that she came to New Orleans to see Elijah, he hasn't returned her calls, nothing. Moments later, Klaus arrives and says, well. "You're abit too late, love. He's in the hands of Marcel, sweetheart. Our brother will be dead, if we don't get him back." Tatia is choked that Family actually means something to Klaus, pretending to be as Katherine/Tatia tells Klaus that she wants to help find Elijah. She asks where Elijah is and Rebekah asks why, she mentions the witch, Davina. Tatia is familar with the name, Katherine says.. Interesting. She's still alive. Klaus realises that Katherine knows this witch, but it is Tatis who does. Klaus asks why, he demands it, but TK decides not to tell Klaus, the truth. Klaus becomes furious. Tatia teases Klaus that she always liked him when he was angry at her, it was why she fell for Elijah, the next day. Klasu & Rebekah are confused, even Hayley is. Right up until, Tatia uses her blood on Hayley and snaps her neck with both of them in shock, Tatia reveals herself to them, but in a way that even Klaus would understand. Tatia says, "Payback's such the B-word, Niklaus. That's what happens when you mess with someone else's bloodline." Rebekah, "Ohmigod!" Klaus, "No! It can't be!" and Tatia ends with, "Oh, but is, Nik. It is me. You see when Mikael died, I woke up, and I knew it was you. I can't wait to see Elijah. I've missed him, but you, Nik. Not so much. But don't worry, your little Wolfgirl will wake up, just she won't be able to get the taste of my blood out of her mouth. The kid's gonna live as well, unlike you, Nik. I actually like children. You don't have much of a choice. Congratulations, by the way. Oh, and one more thing, before I go. Your Mother sends her regards." Tatia leaves Klaus/Rebekah to deal with Hayley on the floor, while Klaus is more confused and pissed at the same time. Rebekah tries to get Klaus to snap out of it. Meanwhile, Tatia finds the whereabouts of Elijah, she stands outside Davina's room and listens to her and Marcel talking about trying to kill Elijah to get back at Klaus and Rebekah. Suddenly, Marcel senses a presense of a unknown vampire outside the room, listening. He opens it and askes who she is, "You're Marcel, right? I'm Katherine. I believe you have an Original in your midst, well, he dumped me, and he's kinda close with my 10th great-granddaughter Elena and I was like, whatever. May I come in?" and Marcel asks why? and T-K says, "Well, I just wanna see him before I leave him in your hands. It's about time, someone stood up to Niklaus. Well done. Oh, you must be Davina, right? The young witch held captive all these years. I'm Katherine Pierce. You heard of me?" and Davia agrees.. of course, what Marcell didn't notice was, Tatia had just compelled Davia to say, Yes without Marcel noticing she did, Tatia does it once more, and Davina invites Tatia inside her room. And before long, the moment she did. tatia grabbed Elijah's coffin and took him out the window and run away with Elijah. Afterwards, Klaus and Rebekah show up and Marcel is pissed at them. Rebekah tells Marcell, "..that wasn't Katherine. It's the one person who made us The Original Family, thanks to our Mother. Now, thanks to you, Nik. She now has Elijah. You should of killed her when, you had the chance." and this is when Klaus says, "I did! The Spirits must've brought her back." Rebekah says, "Or, maybe.. it was Our Mother, Niklaus. It was our Father who turned her, you killed him. SO hell the hell is she still alive?!"" Rebekah and Klaus start argueing and Marcel tells them both to shut up. Meanwhile, Tatia has undaggered Elijah. She waits until he awakes. The next morning, he finally wakes up and the first thing, Elijah sees is that he thinks he is seeing, Katherine. Tatia says, that Elena killed her by giving her the cure and now, she's human, but then she got Damon to turn her again and now she's a vampire again. Just so Tatia can mess with Elijah's mind by pretending to be Katherine. Tatia kisses Elijah, but he pushes away and realises that this is not Katherine. He says, Elena? What have you done with Katherine? And Tatia continues to mess with Elijah's mind and says, "What makes you think I'm Elena, old friend. I gotta admit, I'm totally agreeing with Damon. You were always my favourite Original. So, I'm not Katherine, or is Katerina. If I'm not her or Elena, then who am I? You figured it out, yet. Don't worry, I'll wait. Just tell when?" This is when, Elijah realises, it's Tatia. She says, "Well, by the look on your face. I guess, this is the part where I say.. Hello, Elijah. Did you miss me?! And, this is where you say..." Elijah says, "Hello, My Love." and Tatia ends with, "You always looked good in a suit. I'm sorry about Mikael." and Elijah says, "Thank you. So, what did I miss?" and finally Tatia, "Well, I snapped Hayley's neck... *Elijah is schoked by this, and a little angry* Oops. What, Nik made me mad."

Then, later showed up in TVD by taking Katherine away from MF to NO, just so that she could protect Katherine from Silas. And only this time, Tatia is pretending to be Katherine, after she makes up this story that Katherine found out that she could still e a vampire afterall, while Katherine is been takn care of with Elijah, don't ask why? Just is. Tatia finally runs into Silas, and to her surprise. Silas can't read her thoughts, but has figured out that she is not Katherine. So she tells him that she maynot be Katherine or Elena. But, if Silas wants to see Katherine again.. just so he can drink from her then kill her when hes done with her. She tells Silas that her name is not important, only know that she is in MFs to stay. Because she was HERE first, before Silas. This is her town, not his. Besides, Tatia plans to be rid of Katherine once and for all, since she was the Bad batch.. and Elena is the good one. Tatia sympathises with Silas about how he hates Katherine, but has respect for Elena.