Tasmania enter in 2028. The 20th team will enter in the early 2030s at the earliest. . . Perfect timing for Canberra. I almost guarantee that there're multiple get-out clauses in the ACT's contract with GWS that could get them out early under the right circumstances as well.
Canberrans make up a significant portion of GWS's members, but that doesn't necessarily equate to fans in this case. Most are members because it's a cheaper way to buy tickets and guarantees entry now that their matches regularly sell out.
I know plenty of GWS members, but I honestly can't think of a single person I know that's a GWS fan. Not saying that they don't exist, just that in reality they're very unusual.
At this point GWS playing games in Canberra is effecting their growth in Sydney as well, especially considering how they've been more successful here as a general rule, so it's in both parties interest to bail on the relationship sooner rather than later. Furthermore, whether or not they're viable without a secondary market isn't Canberra's problem, and it's only a matter of time before there's a political change that results in more scepticism about the ACT's relationship with GWS.
Perth, Canberra, or at a stretch Adelaide, are the only realistic options the AFL has on the table, and their decision between them will depend on the direction they intend to take the league in the short-mid term. They'll go with Perth if they want a reliable option that won't require heavy investment, Adelaide if they want to throw another bone to historic clubs and traditions outside of Melbourne, and Canberra if they want to push further growth north of the Barassi line.
The NT would be a shitshow. Political pressure will force them to make a show of considering it, but it won't be in the running unless some crazy funding source is found to bankroll the club, and even then it'd only be a matter of time before that well ran dry.