These are the list of all the possible regnal names (may not be entirely complete, since I may miss some in the way), that the new Pope can and may take as his own regnal name. In parentheses are how the name will look like if the Pope takes the name.

The ones listed below are the names that Popes had been taking since 1775 till today (2013) :

1. Benedict (Benedict XVII) last Benedict still living as Pope Emeritus

2. John Paul (John Paul III) last John Paul died in 2005

3. Paul (Paul VII) last Paul died in 1978

4. John (John XXIV) last John died in 1963, no John before that since 1334

5. Pius (Pius XIII) last Pius died in 1958

6. Leo (Leo XIV) last Leo died in 1903

7. Gregory (Gregory XVII) last Gregory died in 1846

 

Below are the names that the Popes had taken before 1775, but this does not rule them out from being picked by the new Pope, but chances are remote, with the above 7 names in use for the past 238 years.

8. Clement (Clement XV) last Clement died in1775

9. Innocent (Innocent XIV) last Innocent died in 1724

10. Alexander (Alexander IX) last Alexander died in 1691

11. Urban (Urban IX) last Urban died in 1644

12. Sixtus (Sixtus VI) last Sixtus died in 1590

13. Marcellus (Marcellus III) last Marcellus died in 1555

14. Julius (Julius IV) last Julius died in 1555

15. Adrian (Adrian VII) last Adrian died in 1523, last non-Italian Pope before 1978

16. Callixtus (Callixtus IV) last Callixtus died in 1458

17. Nicholas (Nicholas VI) last Nicholas died in 1455

18. Eugene (Eugene V) last Eugene died in 1447

19. Martin (Martin VI) last Martin died in 1431

20. Boniface (Boniface X) last Boniface died in 1404

21. Celestine (Celestine VI) last Celestine died in 1294, also a Saint, the famous Pope who resigned

22. Honorius (Honorius V) last Honorius died in 1287

23. Lucius (Lucius IV) last Lucius died in 1185

24. Anastasius (Anastasius V) last Anastasius died in 1154

25. Eugene (Eugene IV) last Eugene died in 1153

26. Gelasius (Gelasius III) last Gelasius died in 1119

27. Paschal (Paschal III) last Paschal died in 1118

28. Victor (Victor IV) last Victor died in 1087

29. Stephen (Stephen X) last Stephen died in 1058

30. Damasus (Damasus III) last Damasus died in 1048

31. Sylvester (Sylvester IV) last Sylvester died in 1045

32. Sergius (Sergius V) last Sergius died in 1012

33. Agapetus (Agapetus III) last Agapetus died in 955

34. Marinus (Marinus III) last Marinus died in 946

35. Lando/Landus (Landus II) only used once, died in 914, last Pope whose name is only used once

36. Theodore (Theodore III) last Theodore died in 897

37. Romanus (Romanus II) only used once, died in 897

38. Formosus (Formosus II) only used once, died in 896

39. Valentine (Valentine II) only used once, died in 827

40. Zachary (Zachary II) only used once, died in 752

41. Constantine (Constantine II) only used once, died in 715

42. Sisinnius (Sisinnius II) only used once, died in 708

43. Conon (Conon II) only used once, died in 687

44. Agatho (Agatho II) only used once, last one is Saint, died in 681

45. Donus (Donus II) only used once, last one died in 678

46. Adeodatus (Adeodatus III) last Adeodatus died in 676

47. Vitalian (Vitalian II) only used once, died in 672

48. Severinus (Severinus II) only used once, died in 640

49. Sabinian (Sabinian II) only used once, died in 606

50. Pelagius (Pelagius III) last Pelagius died in 590

51. Vigilius (Vigilius II) only used once, died in 555

52. Silverius (Silverius II) only used once, last one is Saint, died in 537

53. Felix (Felix V) last Felix died in 530

 

All the names below have been used only once, ever. Most of them are the early Church Fathers and Saints, and therefore, it is rather extremely unlikely any of these names would be picked.

54. Hormisdas (Hormisdas II) only used once, died in 523

55. Symmachus (Symmachus II) only used once, last one is Saint, died in 514

56. Simplicius (Simplicius II) only used once, last one is Saint, died in 483

57. Hilarius (Hilarius II) only used once, last one is Saint, died in 468

58. Zosimus (Zosimus II) only used once, last one is Saint, died in 418

59. Siricius (Siricius II) only used once, last one is Saint, died in 399

60. Liberius (Liberius II) only used once, died in 366

61. Mark (Mark II) only used once, last one is Saint, died in 336

62. Miltiades (Miltiades II) only used once, last one is Saint, died in 314, First Pope after Edict of Milan

63. Eusebius (Eusebius II) only used once, last one is Saint, died in 310

64. Marcellinus (Marcellinus II) only used once, last one is Saint, died in 304

65. Caius (Caius II) only used once, last one is Saint, died in 296

66. Eutychian (Eutychian II) only used once, last one is Saint, died in 283

67. Dionysius (Dionysius II) only used once, last one is Saint, died in 268

68. Cornelius (Cornelius II) only used once, last one is Saint, died in 253

69. Fabian (Fabian II) only used once, last one is Saint, died in 250

70. Anterus (Anterus II) only used once, last one is Saint, died in 236

71. Pontian (Pontian II) only used once, last one is Saint, died in 235

72. Zephyrinus (Zephyrinus II) only used once, last one is Saint, died in 217

73. Eleuterus (Eleuterus II) only used once, last one is Saint, died in 189

74. Soter (Soter II) only used once, last one is Saint, died in 174 or 175

75. Anicetus (Anicetus II) only used once, last one is Saint, died in 166

76. Hyginus (Hyginus II) only used once, last one is Saint, died in 140 or 142

77. Telesphorus (Telesphorus II) only used once, last one is Saint, died in 136 or 138

78. Evaristus (Evaristus II) only used once, last one is Saint, died in 105 or 107

79. Anacletus (Anacletus II) only used once, last one is Saint, died in 88 or 92

80. Linus (Linus II) only used once, last one is Saint, died in 76 or 79

81. Peter (Peter II) only used once, and he is the founder of the Church, Apostle of Jesus Christ, and it is extremely unlikely any future Pope, or any Pope will ever adopt the name of the Prince of the Apostles.