I think it's quite obvious that Tifa has
something to do with being Cloud's light. The reason why Cloud doesn't want Tifa's light is because he's searching for his darkness - fight off his darkness, which is Sephiroth. People usually seem to assert that with the fact that Aerith is Cloud's light - but what, really? Where is it even shown throughout the game (both I and II), that Aerith is Cloud's light?
First, we have claims saying that Aerith and Cloud meet up in the end, and because all couples are shown to do this as if it's a pattern, Aerith is Cloud's light - big deal, unless Pinocchio and Geppado are in love. And then you see Aerith in KHII
cheering for Cloud and rooting that he'll find his light:
"No matter how far away you are...once you find your light...I'm sure it will lead you back here again. Right?
That seems to be enough of a confirmation that she can't be his light. If KHI proves that Aerith is Cloud's light, then why the hell would she say this in KHII? It doesn't add up - unless Aerith's playing mind tricks on poor Cloud.
And then we have others claiming that Cloud is finding his
inner light. Throughout the game, all it mentions is him battling his
inner darkness. Then there's Tifa saying, "He just needs someone to surround him with light." Why bother adding that line then? In fact, why would you even bother adding Tifa if they were going to make her out to be some kind of stalker chasing Cloud throughout the KH series when she already knows that she's not his light, because her light is too strong for him. Thus, they're not compatible.
But then there's the scene with Tifa confronting Cloud. She glows with a white light, and the next thing we see, Cloud suddenly is glowing with a white light. From that scene itself it should be obvious that Tifa is Cloud's light, or at least, she gave him light for him to fight off Sephiroth, thus them going to a different world (too bad it doesn't end there. And then there's the whole questioning from them asking that if Tifa is Cloud's light, then why wouldn't Cloud come back to thank her...? *facepalms*).
And there's those quotes from the staff members giving blunt hints about who Cloud's light is. You have Nomura saying, if Cloud's darkness is Sephiroth, then Tifa is obviously Cloud's light.
Nomura: In Mr. Nojima's scenario, it explained Cloud and Tifa's connection more in-depth but I deleted it away. I thought it would be more interesting to let the gamers think about it. For example, "If Cloud's darkness is Sephiroth, then Tifa is light"; in that sense you can take it that Tifa isn't really human. The reason Tifa doesn't talk to anyone else besides Sora and Co. may be because she doesn't exist as a human. Of course, I also presented her in a way that she could also be a resident of Hollow Bastion, so I think you can feely think for yourself about her.
If this doesn't prove that Tifa is Cloud's light, I honestly don't know what will to them.
/end rant