I am almost 100% locked on onto Malaysia. It's been 6 seasons and one of the cheapest SE Asia destinations, with just enough to keep it fresh every visit. We could look at Eastern peninsular malaysia (Kuantan/Kota Bharu) or Western/Southern (Ipoh/Malacca/ Penang again why not?) or Kuching/Miri/Kota Kinabalu on Borneo. Our Malaysia fans should keep an eye out this season.
Legs in China are almost always interesting, so count that in for TAR23. They might explore Southern China due to convenient connection by Hong Kong. Cities such as Guangzhou, Xiamen, Haikou/Sanya on Hainan island. If they transit by Shanghai, it would be Suzhou, Hangzhou, Wuxi & the UNESCO Huangshan in neighbouring Anhui province. I do not think they will do a leg in/around the northern part i.e. via Beijing. The only possibility would be Harbin/Shenyang to connect into Russia's eastern edge, or Inner Mongolia/Xinjiang to connect to Mongolia. Count Chengdu out.
Also, Cambodia for the same reasons as Malaysia. Though it would be freaking hot!! The last time it was visited was the Zev & Justin passport disaster leg IIRC. And that was just Phnom Penh. Siem Reap has lots to show in the 10 seasons since its Race visit. And in between the 2 cities, Baray, Kampong Cham, Preah Vihear (UNESCO temple) could be an additional leg in the same way producers like to have Phuket/Chiang Mai as an additional leg when visiting Thailand.
I could envision 2-3 legs in Mongolia/Irkutsk/Lake Baikal region in seasons to come. That is a region yet to be explored. Vladivostok/ROK/Japan might be a stretch due to their proximity to DPRK. Singapore and Philippines are about time too, even if it was just 1 Manila city leg. It would truly surprise me in a good way if the race went to Myanmar, or Nepal.
On Europe: I don't think they can skip this continent entirely. Like Asia, when cultures are so different and in Asia's case when 2/3 of the world resides there, it's not really a "Race around the world" if you don't visit it.
Very low chances of TAR23 going to: France, Germany, Italy, Switzerland
Moderate chances: Spain (least likely Barcelona due to S21), Czech Republic, Sweden, Finland
High chances: Portugal, Hungary, Baltic states, Former Yugoslavia states - you get where i'm going with this
On start/end cities:
Have they ever done Philly?
Just for the random lulz -> Wichita, KS - "aviation capital of US"