There appears to be some interest of late on the topic of who the Grand Duchesses might have married if they had had the chance. Here are some of my thoughts on the matter.
First of all- all of the girls would certainly have made dynastic marriages if explicitly commanded to do so by their parents. Luckily for them, Nicholas was not a forceful kind of person and I think, genuinely wanted his daughters to marry for love. Alexandra always said she wanted the girls to have what she and Nicholas had, but honestly I think she wanted to control them as long as possible.
We know that Olga was considered for several dynastic matches: Prince Carol of Romania, Prince Edward of Wales, and even her father’s cousin Grand Duke Dmitri. Evidently she rejected all of them, and anyhow, Dmitri came to be considered unsuitable due to his wild lifestyle.
In 1913 and 1914, Olga was in love with Pavel Voronov as I discussed in an earlier post. Her parents (personally I believe it was mostly Alix) sabotaged this match. In 1915, Olga begins to mention “Mitya” in her diary. It appears that he was actually Dmitri Shakh-Bagov, an officer from the Caucuses who was treated in the hospital at Tsarskoe Selo. He was born in 1893 and apparently reciprocated Olga’s affection, but the war intervened and he was sent back to the front.
It is unclear whether Olga ever considered marrying Mitya, but it seems likely that marriage was probably on her mind. In 1915, she was already 20 years old. Her cousin Irina, who was only a few months older than she was, was already married with a child. Olga was especially close to her Aunt Olga Alexandrovna, who in 1916 was granted a divorce from Peter of Oldenburg so she could marry Colonel Nicholas Kulikovsky. She mentions in a letter from captivity in Tsarskoe Selo that she regrets not having had a “heart to heart” with her aunt at their last meeting. Could she have wanted to discuss her love life? We will never know for sure, but I think that based on Olga’s track record, she would have probably married an officer and really been quite happy.

The gentleman with his head circled between Olga and Tatiana is Mitya.
During the war, Tatiana met a young soldier named Dmitri Malama while working in the hospital. She treated his injuries and evidently he admired her, because he gave her her dog Ortino (the first one) as a gift. They were by no means a “couple,” in modern terms, but Alexandra commented in a 1916 letter to Nicholas “I must say, a perfect son-in-law he would have been-why are foreign princes not as nice? Ortino [the dog] had to be shown to his ‘father’ of course.” Malama was born in 1891 and apparently when he learned of the murder of the Imperial Family, he went out of his way to be killed in action fighting for the Whites in 1919. I think that Tatiana would have liked to marry Malama, and he certainly cared for her. It’s even possible that Alexandra might have allowed the match, had things been different. (Tatiana was her favorite, after all.) However, I do think that Tatiana would have been most likely to make a dynastic marriage without complaint simply to make her parents happy.

Tatiana standing next to Dmitri Malama’s hospital bed. Olga is sitting near his feet.
Maria always wanted to marry an officer and have many children. I think that she would have followed her sister Olga’s hypothetical footsteps and married a commoner. She was a great flirt and had a crush on Nicholas Demenkov. She often signed her letters to her father playfully with “Mrs. Demenkov.” Maria was very gentle and kind and would have made a wonderful wife and mother.

Demenkov is the man directly behind Tatiana, with the mustache.
With Anastasia, it is difficult to tell. She was really too young to have a particular “sweetheart” and never had one to anyone’s knowledge. I think that she would have been just as happy to marry a commoner as to make a dynastic marriage, as long as it was for love. Who knows, maybe Anastasia did not want to get married.
I think all of the girls would have made wonderful wives and mothers and it is a shame that they never experienced those parts of life and that consequently we will never know about them!
Sources: The Romanovs: Love, Power, and Tragedy by Dr. Manfred Knof, Diary of Grand Duchess Olga Nicholaevna, 1913, A Lifelong Passion by Andrei Maylunas and Sergei Mironenko, and The Alexander Palace Time Machine Forum