I find that one major point of disagreement i have with most people in forming these hypothetical XIs is i approach it with 'team balance' as a major factor while others approach this with 'hmm. who fits in this slot the best?'.
For example, I'd still pick Mankad to partner Gavaskar in an alltime India XI- despite Sehwag having much better stats as a batsman (though if he keeps up like this for 20-30 more tests, he'd edge out Mankad). Simply because, IND's bowling doesnt add up very well and Mankad's bowling(he was a left arm spinner if i am not mistaken) would give the team an added flexibility.
Similarly, I consider having 5 bowlers a must for most alltime XIs, with the 5th being an allrounder who i'd even forgive for having merely worldclass bowling and not great bowling.
And Kallis, IMO, is a very ordinary bowler. If he was a bowler alone, he'd be the Venkatesh Prasad of South Africa. Ofcourse you can argue he has more wickets/better average but the point you'd be missing is that Kallis bowled almost exclusively in highly bowler friendly conditions and the bowling cast around him had impeccable pedigree.Having being a bowler, i know that bowlers operate as a unit far better than batsmen do, who are infact playing an individual game (whether they arebeing selfish or looking at team interest is not the issue here- they are doing it completely individually). True, good batting lineup matters in terms of releiving pressure on a particular batsman but its nowhere as important as having a good bowling lineup if you look at how it impacts particular players.
In that aspect, Imran would be the most logical choice. Other excellent and much better batting candidates exist but think on this- The lineup already has brian,sachin, ponting, waggy and hayden, with the wicketkeeping spot a lock for Gillchrist.
Now, i dont consider gillchrist an alltime great batsman of the same callibre as Steve Waugh or Viv but he is absolutely worldclass (in my world of heirarchies, worldclass is one notch below alltime great). There is no point in shoving in another excellent batsman with very mediocre/no bowling when the team would benifit far more from having 5 competent bowlers.
Just my two cents.