A state visit from Sweden (my homecountry) will based on the current practice see Charles recive the Collar of the Royal Order of the Seraphim (he’s already a Knight since 1975), Camilla become a member of the Royal Order of the Seraphim, William become a Knight of the Royal Order of the Seraphim (without the collar) and Catherine become a Member Grand Cross of the Royal Order of the Polar Star.
The King of Sweden is already as could be seen on monday, a Knight of the Order of the Garter and a recipient och the Royal Victorian Chain… And if UK continues with their tradition of not give any orders to spouses of the Head of State, i expect no new orders in that direction…
If the planned incoming state visit from Japan that was cancelled during the pandemic is rescheduled, it will see Charles recive the Collar of the Supreme Order of the Chrysantemum (he’s already a Knight since 1971), Camilla become a Grand Cordon of the Order of the Precious Crown, William become a Grand Cordon of the Supreme Order of the Chrysantemum (without the collar) and Catherine become a Grand Cordon of the Order of the Precious Crown.
The Emperor of Japan will most likely in Return be made a Supernumerary Knight of the Order of the Garter.
Thanks for your excellent information about Sweden and Japan. Here are some state visits that I think might happen in the next years and possible associated honors:
1) Outgoing visit to France: the King is already a Grand Cross of the Order of Legion of Honour and the Queen Consort is already a Grand Cross of the Order of National Merit, so I expect no new awards.
2) Outgoing visit to Germany: I believe neither the King nor the Queen Consort hold any German honor, so I expect the Grand Cross of the Order of Merit of the Federal Republic of Germany to be awarded to both of them.
3) Outgoing visit to Spain: that is a difficult one. The King already has the Grand Cross of the Spanish Order of Charles III; I expect him to be made a Knight of the Golden Fleece (which is by personal discretion of the Spanish monarch) , but the Spanish government may also approve the awarding of the Collar of the Order of Charles III to the British King and of the Grand Cross of the same order to the Queen Consort. Alternatively, the Queen Consort may be given the lower Grand Cross of the Order of Isabella the Catholic, but I find that unlikely.
4) Outgoing visit to the Netherlands: the King already has the Grand Cross of the Order of Orange-Nassau and the Grand Cross of the Order of the Crown; I expect that both the King and the Queen Consort will be further awarded the Grand Cross of the Order of the Netherlands Lion.
5) Incoming visit from Belgium: apparently neither the King nor the Queen Consort have any Belgian order. If there is an incoming visit by King Philippe and Queen Mathilde, I expect King Philippe to award the Grand Cordon of the Order of Leopold to King Charles III, to the Queen Consort, and possibly also to the Prince of Wales; the Princess of Wales will probably be awarded the Grand Cross of the Order of the Crown.
6) Incoming (?) visit from Sweden: if there is an incoming visit from King Carl XVI Gustaf and Queen Silvia, I concur that the most likely scenario is for the Swedish monarch to award the Collar of the Order of the Seraphim to King Charles III (who is already a Knight) and extend membership in the same order (without collar) to the Queen Consort and the Prince of Wales; the Princess of Wales should get the Grand Cross of the Order of the Polar Star.
I don't expect incoming or outgoing visits from/to Denmark and Norway, but the King is already a Knight of the Order of the Elephant and a Knight Grand Cross with Collar of the Order of St Olav, so any new awards would be for the Queen Consort only, and maybe the Prince of Wales.
On the British side, I have a suspicion that King Charles III will abandon his mother's precedent (if the courtiers approve it) and start decorating queens consort. In that scenario, I expect Queens Letizia, Maxima, Silvia and Mathilde to be made Honorary Dames Grand Cross of the Royal Victorian Order, but that may be wishful thinking on my part! Kings Felipe, Willem-Alexander, Carl Gustaf and Harald, and Queen Margrethe are already Knights/Lady of the Garter and I expect King Philippe to become a Knight of the Garter too if there is an incoming Belgian state visit. President Macron might become a Honorary Knight Grand Cross of the Order of the Bath.