Possible wives for Alexei: (b.1904)
**Princess Margarita of Greece (b.1905)
**Princess Theodora of Greece (b.1906)
**Princess Irene of Greece (b.1904), dau. of Constantine and Sophie
**Princess Sibylle of Saxe-Coburg-Gotha (b.1908), dau. of Charles Edward, Duke of S-C-G
**May of Teck/Athlone (b.1906), dau of Princess Alice, Countess of Athlone and the Earl of Athlone/Prince of Teck
I think that, due to Alexei's condition, his parents would have picked a wife from among his cousins, (although this may not have been the wisest choice), most likely from Alexandra's side of the family, as she did not like many of the Russians. The wife would probably have had to been "English" in breeding. Any of the above girls would have fit the bill, although May of Teck might have been a disastrous choice due to the haemophilia aspect.
But I agree with earlier posters, I think Ileana of Roumania (b.1908) may have been the most likely choice. I believe she and Alexei had been sweet on each other and when they were older, say 18 and 22, the age difference would not have mattered at all. She was "English" in breeding and already Orthodox. And, earlier, a marriage between Ileana's brother Carol and Alexei's sister Olga had been thought of, when the family sailed to Roumania for the young couple to meet but they apparently didn't get along. So, Missy's children may have already been "approved marriage material" in the minds of Nicky and Alix. I'm sure the lineage and upbringing were fairly acceptable to Alix, even if she and Missy were opposite poles.
Fun to think "what if"....