I'm usually pretty good at reading the producers' minds - a few seasons back I got every country right.
Central America as the penultimate leg I feel is more likely South America, unless they're going to skip Brazil and Argentina for the first time. Much as I'd love to see the Caribbean islands (especially the Dominican Republic) or Mexico I think they're both unlikely - Costa Rica is the best bet in this region.
Since season 17, we've done Norway, Sweden, Austria, Liechtenstein, Switzerland, Denmark, Belgium, Italy, Germany, the Netherlands, Spain, France, and the UK (with Germany hit three times and the Netherlands twice). That's every western European country except Iceland, Ireland, Portugal, Andorra, and Luxembourg. In the same time period, the only eastern European country visited is Russia (twice). If we go to Europe I'd put it at about 95% likely it's the eastern half, and probably somewhere in the former Yugoslavia given all of the regional franchises COMBINED have only spent two legs there (TAR12 in Dubrovnik, and the HMLM2 leg covering both Sarajevo and Dubrovnik). Or Greece, if they can get in and out safely - though with Golden Dawn's political influence, that's unlikely if there are going to be minority teams.
Africa? Most likely Morocco if anywhere - easy connections to Europe (especially to Portugal), a safe country, not the same landscape as the Africa seen in the last three visits (Malawi, Tanzania, Botswana) - although Angola is an outside chance, and I'd love another leg in South Africa if they can do it. The Arab Spring countries are probably still out at least until TAR24, and the remaining countries are either too similar to what we've seen recently (Gabon) or not safe (Kenya). It's definitely time for a "big city" leg in Africa again, and preferably one where "poverty LOL" isn't the focus. Maybe Ethiopia?
Unless the show is given a midseason "another show isn't rating" renewal we've only got one season this time around, so India is probably the safest bet you can make, but aside from that Asia is tougher. Middle East is still out (Jordan is the most plausible option, and it has borders with Iraq, Saudi Arabia, Israel, Palestine, and Syria, which should tell you all you need to know), Japan and Korea are probably on the blacklist until Kim Jong-Un calms down, Indonesia needs a break, and we had Hanoi this season. At a guess I'd say Malaysia or the Philippines. If they're going to insist on pointing out the lack of originality in recent seasons by having another Switchback, then the Philippines is DEFINITELY the better option.
No to New Zealand and the rest of the South Pacific islands, and Australia is an outside chance at best. If they do, it'll be somewhere that isn't beachside or "outback", and the only real options that are race-friendly are Victoria and Tasmania (and Adelaide, but pfft).
So in summary, my guess:
United States
Portugal
Morocco
Serbia
Russia
India
Australia
Philippines
Costa Rica
United States
That's nine countries, with one new - roughly the same as this season. If I had to guess, I'd say the extra three legs come from double legs in Morocco (because the show hasn't had a single leg in any African country for twelve seasons), Australia, and the Philippines.