Let's suppose that Elizabeth had taken an active role alongside the plotters of Wyatt's Rebellion -if Queen Mary had married King Philip of Spain they would have conceived a child, and that would have presented a threat to Elizabeth, who would then have an interest in preventing it from happening.
Supposing that she had been found guilty and executed, how would the absence of a Queen Elizabeth had affected the future of England? Would it have remained Catholic?
Mary was not ever going to have a child. She was suffering from ovarian cancer by time she became Queen. Conception would be impossible. Philip was also only in England for two brief periods. In 1553-1554, and later in 1557. There's only two small gaps for her to even conceive had she been healthy and able too. Mary also suffered from irregular periods and the issue where fluids from the discharge would be maintained within the body instead of coming out naturally. I think she might have a hard time conceiving. She always had ill health.
If Elizabeth was executed, Mary would stick to the Will of Henry VIII that regulated the succession. With Elizabeth gone, the succession passes to the heirs of his sister Mary Tudor. That included Frances Brandon, Duchess of Suffolk, and her two surviving daughters, Catherine Grey and Mary Grey. Neither girl were very bright, and Mary was hunched back. After that came Margaret Douglas, I believe, who had rights as she was born in England. Mary actually wanted to name Margaret her heir as she knew she'd keep the Catholic religion. We'd get a fun situation with the Lennox Stuarts reigning in England and the main branch in Scotland. Darnley as Prince of Wales, perhaps married to Mary Stuart... but given their messy martial situation, I dunno if would still be considered. It was though, we could see a union of the crowns in 1578 where Darnley becomes King of England with Mary as Queen of Scotland and his consort.