WI: Kaiser Wilhelm is 1st in line for British Throne.

[Steyr]

Due to this being in a time period where the idea of authoritarian Germany having a personal union over a democratic Constitutional Monarchy in Britain scares literally every British person, Parliament would either abolish the monarchy out of fear of German dominance or choose a son of Wilhelm to take the throne instead. Personal unions just aren't a thing that could last between two major powers in the 20th century.

 

[Mikestone8]

One more thought. Might it be simpler to pass an Act nominating one of the Kaiser's sisters?

Due to Salic Law they could never inherit Prussia, so that forecloses the matter unless another German Crown Prince marries another British Princess Royal - which is highly unlikely.

 

[QueenMaud]

In addition, there's a third sister Victoria, who'd be 3 at the time.

By 1892, when George & Eddy bite the dust, IOTL Louise was already married to the Duke of Fife and, depending on butterflies, already have a one-year-old daughter Alexandra (they also had a stillborn son in 1890, but given the butterflies, Louise & Fife could have completely different kids). Toria, who was a spinster IOTL because Queen Alexandra insisted on keeping her as a companion, likely gets married to secure the succession.

The only chance one of Willy's descendants ends up on the British throne is as a consort to Louise's kids, either Victoria Louise (or an ATL sister) if Louise has a surviving son ITTL, or one of the younger boys for her OTL eldest daughter Alexandra. Even that is unlikely as at the time of the marriage Queen Victoria, the biggest cheerleader for the match, is dead and their Danish grandmother is alive and won't allow that marriage over her dead body. Alexandra of Denmark (and Edward VII) despised Willy, and to her, the German kids are the spawn of Prussia (who stole Schleswig Holstein from Denmark) and the German family that took over the Schleswig Holstein title. The Wales girls will be pretty deferential to their parents, dad died young and thus lives on in eternal nostalgia, and mom was the only one there and already over-protective IOTL. If Alix tells her daughters "Don't to marry off my sweet grandchildren to dirty Prussians, especially Willy's children, who your late father disliked" the wedding won't happen.

They're much more likely to get married to a different cousin if Louise only has girls the best choice might be a non-hemophiliac son of Princess Beatrice and Henry of Battenburg (either Alexander or Maurice). They're close in age, related, and (importantly for Alexandra) German in name only, and even then they come from a morganatic Hessian branch, and Hesse hated Prussia as much as Denmark did. They're basically the male equivalents of Mary of Teck: fathers born of morganatic marriages, mothers British princesses, and both raised in Britain. If the heir's a boy (say a surviving Alistair of Fife, probably named Albert Edward ITTL for his late maternal grandfather), Alexandra likely pushes for one of her nieces or a descendant of Princess Alice (Hesse was one of the few acceptable German houses to Alice, especially because the Hesse girls were raised by Queen Victoria,). Options include Dagmar of Denmark (Alexandra's Danish niece), Marie or Stephanie of Schaumberg Lippe (grand-nieces of Alexandra), Margaretha of Sweden (another grand-niece of Alexandra), Louise of Battenberg (Grandaughter of Princess Alice, IOTL Queen of Sweden & Aunt of Prince Philip, Duke of Edinburgh).

About Louise - she could marry one of the Swedish princes (oscar or carlos) since in the 1880s there were rumors of a marriage between Louise and Oscar that obviously did not happen because of the prince's determination to marry a Swedish noblewoman. As future Queen of the UK I find it unlikely a marriage with Macduff.
If Louise and her Swedish husband had a son a marriage to Princess Dagmar of Denmark (1890) could be encouraged by Alix, although Victoria Louise of Prussia would certainly be considered. In addition they would have the great Russian duchesses of similar ages.
If Louise had a daughter Prince Gustav of Denmark (1887) or Prince Wilhelm of Sweden (1884) could be suitable options

 

[QueenMaud]

Speaking of marriages, the shortage of British princesses might raise the market rates of the auxiliary branches (namely female-line descendants of Queen Victoria like Helena Victoria & Marie Louise of Schleswig Holstein (Lenchen's daughters), Victoria Eugenia of Battenberg (Beatrice's daughter and OTL's Queen of Spain), Alice & Louise of Battenberg (great-granddaughters via Alice's eldest daughter Victoria, IOTL Prince Philip's mother and aunt who was also Queen of Sweden), and even the auxiliary to the auxiliary May of Teck (OTL's Queen Mary whose mom was Victoria's cousin).

Marie Louise of Schleswig-Holstein was considered by Wilheim II as a possible consort for Ferdinand of Romania so they could marry in 1891 I imagine Thora could marry Ernest Louis of Hesse Mary of Teck was certain to marry some Russian grand duke, her father ignored the Grand Duke Mikhail Mikhailovich (1861) when he proposed to May because the family wanted the princess to marry Eddy / George, so she could marry him , or Paul Alexandrovich (1860) after the death of Aline in 1891
One of Battenberg's sisters could marry the duke of galliera while the other could marry some russian drand duke (Boris wanted to marry Victoria Eugenia so he could marry Alice/Louise)
Ernest II of Hohenlohe-Langenburg was considered to Toria but I do not believe that Alix would allow such a marriage, certainly Christian f Denmark, Max of Baden, or Wilhelm of Luxembourg could be more appropriate
Kiril would certainly marry some smaller German princess, perhaps one of Vera Constantinovna's daughters could be considered
Gustav VI Adolf could marry Maria Pavlovna, certainly more appropriate than a princess of Battenberg