I'd spend most of my time getting the basic outlines of the other factions set up. Basic banners, color combinations for the factions, names for characters, that sort of stuff. And help on figuring out where to put the other cities. Then start putting them on the map in their appropriate places. 50-21=29, so I'd like to add:

22. Pergamon
23. Bosphorian Kingdom
24. Syrakousai
25. Massaeili
26. Celtiberia
27. Kimbrii
28. Chattii
29. Erain
30. Boii
31. Bithynian League
32. Aitolian League
33. nascent Achaean League
34. Some type of Ethiopian faction
35. Another South Arabian faction
36. A way to get an Indian faction in
37. Emergent Yuezhi faction
38. Massagetae
39. Iberian faction (not spain-iberia)
40. Lugians
41. Sinope-Trapezous faction
42. Another Celtic faction in Gaul
43. Massalia faction
44. Cyrene
45. Some sort of Berber or Garamantene faction
46. Division of Sauromatae faction into two tribes (or three)
47. Illyrians
48. Kappadocian faction (if more provinces)
49. Emergent Judaean faction
50. Emergent Rival Baktrian faction