HKG and SIN are unlikely due to no First. Having said that, if multiple daily frequencies are flown, the A350 could be utilised alongside a 777 or A380 and First maintained on the route. Not in the Far East, but I could see SFO or SEA working like this.
As for BKK: this could be more likely but I doubt that it will be prioritised.

For those asking about the A350 to the US and for the Summer 2020. I’ve discovered something that I’m sure is probably right but not confirmed yet. After the 29th of March 2020 the current A350 routes including Dubai, tel Aviv and Bangalore switch to a 3 class 777 (which Is a bit odd) along with these Philadelphia and Washington DC also switch to a 3 class 777 in fact, Philadelphia’s first flight switches to a 3 class 777 which is currently being served by a high j 747 so that in my opinion just has to be an A350. Many of These routes have never been served by a 3 class 777. Also the Sunday 189 to EWR is a 3 class 777 from 29th March, from 1st June Delhi is a 3 class 777 And from 15th June ABU DHABI is a 3 class 777. Heathrow doesn’t even have enough 3 class 777s to fly all those routes. So I think my theory holds some truth. But Let’s see

Definitely A350

BAs A350 arriving at LHR in the afternoon of 26 July.