Even at 33, I doubt Mary could have children.. although she came to the throne IOTL at 37 and was quite old (at the time) for marriage and children, it wasn't impossible. It was simply the ovarian cancer that killed her. We can never pinpoint when exactly it developed, but I wouldn't be surprised if she was already in the early stages in 1549.
There is also the issue that Mary was plagued with gynecological problems throughout of her life, such as irregular periods and even issues with fluid being retained that caused great physical pain. Given her assortment of gynecological problems in her later life, children seem quite unlikely IMO.
As for her succession, I could imagine some worry, seeing her succeed in 1549, barely two years after her father's death, with her brother's short reign cosigned to the dustbin. Mary has one advantage of succeeding at this time, as the Edwardian government had not yet began to radicalize things, so she will inherit a church still close to the Henrican Acts and more conservative. The faith of the English is still muddled, but not muddled between Catholics, Henrican Anglicans, and those who favored Edwards reforms. In this scenario, Mary is definitely in a better position, as while the common people are "confused," the Henrican Establishment is still strong and returning to Rome is not as farfetched. There will be no Jane Grey; I forsee Mary having no problems with her ascension.
Still, there would still be opposition from those who have no desire to return to Rome. Radical religious ideas have still been introduced from abroad, whose fans could be stroked if Mary is particularly ruthless in attempting to restore England to Rome. In an interesting twist, it is Elizabeth who these Protestants might look too as their figurehead.
Marriage is also an interesting twist. In 1549, Philip II is of course unmarried, his wife having died in 1545. So they might well marry, as Mary was slavishly dependent upon him and he would take advantage of it. There are other options, such as a Portuguese Infante, like
Dom Luis, the Duke of Beja. As I said earlier, Mary may still be suffering some early stage ovarian cancer, so children may be impossible, no matter who she marries (which she will -- unlikely Elizabeth, she saw a desire to marry and definitely would).
Domestically, Mary may have a better reign with more time to handle things. She'll be inheriting a war in Scotland (the "Rough Wooing") which she'll most certainly try to end ASAP... unless she is urged on by the Emperor to continue with Spanish aid, ostensibly to remove the French from Scotland. It's more likely that Emperor Charles mediates to end the conflicts. This gives Mary a chance to focus on restoring the economy. IOTL, this was one of her many achievements, seeking out new ports, working on strengthening the pound, ect, yet a achievement that English was able to take as her own as the economy recovered on her reign.
Other issues include the monastatic lands, the more recent Chanteries Dissolutions (which effectively provided education for poor children and in rural areas), the constant devaluation of the pound, and relations with France.