I think that some of the recent suggestions in this thread are not iconic enough. It will be two armies that are versatile enough in their backgrounds to be fighting all over the old world (or more importantly a couple of books shoved under a green mat). I very much doubt they will stick tomb kings in as a starter as they don't really fit, they are based in khemri and have no real business fighting in the northern wastes or the worlds edge mountains, the counter argument to this is that GW used lizard men as a starter force and this army suffers in exactly the same way (why were they fighting Bretonians!?) but from what i recall it was not a popular decision and I doubt GW will do the same again.
As for Dark vs High elves I think these armies would be far too similar in look and list combination so this would also be a no no. They will pick to armies with a stark, different look and playing quality, that have different strengths and weaknesses.
Empire is a good shout for a possibility, They have not been in a starter set before which seems strange as a lot of the fluff is written from their perspective. Who they fight is more difficult. OaG is a really great starter army but has already been used in 2 editions. CW are just evil men and i would expect two entirely different races. Skaven are just having a re-release now which I think puts them out the running. I agree with previous comments that VC have too much of a specialised playing style for a starter set but i think empire and VC would be a cool combo. It would look great and there are a million different reasons why the two would come in to contact all over the old world,
Personally my ideal would be High Elf / Chaos. They are the antithesis of each other and there is nothing like pitting tough close combat powerhouses against agile and nimble foes with excellent twangage! Plus the magic phase when these two meet is always something else!
thats my 2 cents.