Это еще вилами по воде писано
I do not believe it will come true, it is hardly probable. See Бабушка еще надвое сказала (Б)
Cf: If the sky falls, we shall catch larks (Am.). It is still quite in the air (Br.). That remains to be seen (Am.). There is many a slip twixt cup and lip (Am.). There's many a slip between ('twixt) the cup and the lip (Br.). We shall catch larks if (when) the sky falls (Br.)
Русско-английский словарь пословиц и поговорок.
Академик.ру.
2013.