Endometrial Hyperplasia with Fibroid: The presence of abnormal endometrial thickening (24mm) and a mucosal fibroid suggests a possible diagnosis of endometrial hyperplasia, which can be associated with fibroids. The thickening and fibroid could be causing the abnormal ultrasound findings.

Large Endometrial Polyp: Given the description of a possible large endometrial polyp, this is a plausible diagnosis. Endometrial polyps can cause irregular menstrual bleeding and can be associated with endometrial thickening.
Uterine Leiomyoma (Fibroid) with Secondary Changes: The mention of a mucosal fibroid suggests that the fibroid could be causing the endometrial thickening, possibly due to secondary changes such as inflammation or pressure effects on the endometrium.
Complex Endometrial Hyperplasia with Atypia: Although less common, the presence of a complex cyst and significant endometrial thickening raises the possibility of complex atypical endometrial hyperplasia, which has a higher risk of progressing to endometrial carcinoma.

Endometrial Carcinoma: This is a critical diagnosis not to miss, as it can present with endometrial thickening and complex cystic changes. The presence of a large polyp or fibroid does not rule out the possibility of underlying malignancy.
Sarcoma (Uterine Leiomyosarcoma): Although rare, uterine sarcomas can present with rapid uterine enlargement and complex cystic changes, making them a "do not miss" diagnosis due to their aggressive nature and poor prognosis.

Adenomyosis with Cystic Changes: While adenomyosis can cause uterine enlargement and cystic changes within the myometrium, it is less commonly associated with significant endometrial thickening and complex cysts as described.
Müllerian Duct Anomalies with Secondary Changes: Certain müllerian duct anomalies can lead to complex uterine structures and cystic changes, but these would typically be identified earlier in life and are less likely to present de novo in a 43-year-old woman.