I know some of my Indian friends here disagree, but I completely agree with Sanjay Manjrekar when he says Sachin and Zaheer should be preserved for South Africa 2013. That's our next big target, and we should start preparing for it right away! And the first step of that preparation would be to sit down and have a chat with both Tendulkar and Zaheer and ask them if they are also on the same page. If they are not, it would be time to shake hands with them right now and move on amicably.
Sachin still has a bit of cricket to offer. With a highly formidable pace attack that South Africa possess right now, throwing in all fresh guys (only Kohli has had a taste of high quality fast bowling in overseas tests) at the #1 team in the world in their own backyard could be disastrous. Having one guy in there who has had that experience and who can lead the other guys around him is a must. Tendulkar is that guy. If he says right now that he is not sure about his future plans, then the selectors must ask him to announce retirement now or take the most drastic step of DROPPING HIM (which has never happened in Indian cricket history).
With Zaheer, he will be 34+ by the time we reach South Africa next year. For a fast bowler, that's the twilight of his career considering that most fast bowlers retire around that time. He should also be told that the South Africa tour is the most important one and if he expects to last till then, he should be told to preserve himself or else back out so that other fast bowlers can be readied.
As far as Sehwag and Gambhir go, I won't be surprised to see two completely new openers opening for India in South Africa. Gambhir's test form has dipped alarmingly and for long now. And Sehwag as we all know has only one method of dealing with bowlers - my way or the highway!