The ones I'd like to see, in order of importance to me:
1) The Seleucid Empire (this one should be obvious)
2) Celtiberians/Arevaci (possibly the best starting position in Iberia, along with the widest variety of units there)
3) Dacia (as obvious as the Seleucids, really)
4) Scordisci (a unique mix of Celts and Illyrians, in an interesting location to boot)
5) Pergamon (the forgotten Successors need to have their story told)
6) Galatians (Celtic madness in Anatolia, what's not to like?)
7) Gallaeci (or Cantabrians, but preferably Gallaeci because of their location)
8) Boii (long live the Eastern Celtic Empire!)
9) Armenia (one of my favourites in the old RTW)
10) Belgae (the bigger brother of the Iceni, and with a more interesting starting position)
11) Veneti (SHIPS.)
12) Helvetii (CELTIC PHALANXES.)
13) Sarmatians (the steppe peoples are under-represented at the moment)
14) Any little
Thracian tribe still alive (because... well because RHOMPHAIAS.

)
15) Bactria (not a personal favourite, but I figured they should not be left out)