"More serious" can be replaced with several synonyms depending on the context of the sentence. One option is "graver," which implies a more severe or critical situation. Another synonym could be "severe," indicating a harsh or demanding situation. "Dire" is another option, indicating an extremely urgent or dangerous circumstance. A less common but still valid replacement could be "solemn," conveying a sense of gravity or importance. Another possibility might be "weighty," suggesting a situation of significant consequence or impact. Ultimately, the synonym chosen will depend on the specific context of the sentence, as well as the speaker or writer's intended emphasis.