Here are all the countries which have still never won the ESC, with their debut year in brackets:
Portugal (1964)
Malta (1971)
Morocco (1980)
Cyprus (1981)
Iceland (1986)
Slovenia (1993)
Bosnia & Herzegovina (1993)
Croatia (1993)
Romania (1994)
Slovakia (1994)
Lithuania (1994)
Hungary (1994)
Poland (1994)
F.Y.R. Macedonia (1998)
Belarus (2004)
Andorra (2004)
Albania (2004)
Moldova (2005)
Bulgaria (2005)
Armenia (2006)
Georgia (2007)
Montenegro (2007)
Czech Republic (2007)
San Marino (2008)
I've crossed out the countries that are not taking part in ESC 2015. So out of the 24 countries listed above that have yet to win the ESC, there are 18 possible ESC 2015 winners.
Bear in mind though, it's now 21 years since 7 new countries (Estonia, Romania, Slovakia, Lithuania, Hungary, Russia, Poland) had their debut entries in ESC 1994. Two of those have since won:
2001 Estonia - 7 years (1 x 7) after 1994
2008 Russia - 14 years (2 x 7) after 1994
Under that logic, now that it's 2015, i.e. 21 years (3 x 7) one of the remaining five counties (Romania, Slovakia, Lithuania, Hungary, Poland) from the 1994 debuants is due to win this year. Slovakia isn't taking part this year, so that leaves
Romania, Lithuania, Hungary, Poland as likely winners under this logic.