Seriously though, off the top of my head, I could easily think of enough to get to 50 (9 in the Expansion, 7 as DLC):
35. Poland
36. Assyria
37. Portugal
38. Zulu
39. Majapahit Indonesia (more marketable, amalgation civ)
40. Khmer Israel (Khmer has too big an overlap with Thailand, if we go for 50 civs)
41. Kongo
42. Brazil
43. Vietnam
44. Mali too big an overlap with Songhai, I'd say "another African civ": Nigeria, Ashanti, Benin, Sudan, Uganda (Idi Amin!) OR Tibet/Modern option from 49. & 50.
45. Sumeria
46. Hittites
47. The Moors Morocco (sounds better than Moors)
48. A second Native North American Civilization (i.e. Cherokee, Inuit, Hopi, Sioux, Apache, Navajo, Comanche)
49 & 50. Two modern nations, i.e. Italy, Belgium, Australia, Canada, Argentinia, Colombia, Cuba, Bulgaria, Romania, Hungary, "Serbia/Yugoslavia" (Tito!), Armenia which would be quite marketable
51.+ Other options include "Non-Modern Era exotic civs" like Maurya, Chola, Chachapoya, Tupi, Tibet, Goths, Etruscans, Seleucids, Lithuanians, etc. ...
That's before going into some of the controversial options that they likely couldn't do.