The options for team 20 that get discussed in Aussie Rules circles are Canberra, NT/NT and Cairns in a "Northern Australia team", Perth, Adelaide, Auckland, and Brisbane as a possible smokey.
You can basically rule the NT, NZ, and Brisbane out. Despite the popular media support for it, the NT/Northern Australia stuff is a pipedream. If Auckland/NZ is explored as an option it will almost certainly fall apart in the early stages of business planning unless there's significantly more broadcaster interest for AFL content in NZ than anyone reasonable suspects. And though the AFL wants a second Brisbane side to happen, and will almost certainly look at it behind the scenes, a second Brisbane side would be even more expensive and less stable than GWS and the Suns at the moment, and there just isn't any will for another project like at the moment.
That leaves Canberra, Perth, and Adelaide as the only realistic options.
The GWS deal isn't a major concern or road block to Canberra getting it's own side as pretty much everyone who pays attention can see that Canberra's situation with GWS is becoming increasingly toxic. Both sides of the fanbase are becoming increasingly resentful of the other, and as a result this current deal will almost certainly be the last no matter what happens.
The real road blocks to Canberra getting it's own side is the lack of an AFL standard stadium in Canberra, and the fact that it'd almost certainly be another expensive expansion project that would need to be heavily underwritten to survive just like GWS, the Suns, Lions, and now Tasmania.
Adelaide and Perth, especially Perth, are seen as markets where you could plonk a team down tomorrow and it'd more or less support it's self from day dot without the need for much interference or support from the AFL, and that's a very attractive prospect to a lot of the AFL's stakeholders, especially the other clubs.
Anyway, that's basically the lowdown of the current AFL expansion debate, and currently I'd put my money on Perth getting the 20th team if I was a betting man.