Following the death of her husband the Emperor Henry V, the Empress Matilda set out to return to the court of her father, King Henry of England. The death of her brother William Adelin five years before had left her the presumptive heiress to her father’s possessions, England and Normandy. She was thus the greatest heiress in all of Christendom. Fearful of being kidnapped and forcibly married, she passed into the lands of her cousin Count Charles of Flanders, from where she could pass into Normandy or England with ease. She was still there as envoys arrived, hot on her steps, from the German lords offering the German crown to Charles. Ever domineering and ambitious, Matilda pressured Charles into accepting, with her as his chief supporter and new wife.
The marriage maintained Matilda’s Imperial dignity and strengthened Charles’ position somewhat, particularly given the promise of inheriting independent powerbases in Normandy and England and creating a mighty North Sea empire. Opposition soon rose up all sides - the Pope clamoured against Matilda and Charles’ consanguineous marriage and Matilda’s cousin William Clito stirred up rebellion in Normandy, from where he might well usurp the English throne once her father died. Elsewhere the mighty Duke Lothair of Saxony was “elected” by his supporters while Henry V’s nephews the Hohenstaufens laid claim to various crown lands acquired by their uncle and grandfather.
At length Pope Honorius II, previously a Papal legate to Henry V’s court, granted the necessary dispensation and recognized Charles as German King, desiring as weak an Emperor as possible. Lothair of Saxony defeated Charles in battle but was forced to turn his attention to the far greater Hohenstaufen threat amassing at his rear. The Hohenstaufens and Charles ally, Charles granting them most of their requests in return for aid against Lothair. By the Treaty of Ghent Charles betrothed his infant son, Henry, to Bertha of Swabia, pardoning the Hohenstaufen brothers and restoring them to their lands. The infant soon dies and Charles the Good is assassinated by Flemish agents of Lothar.
Isolated in Flanders, Matilda betroths her young daughter Richtilde to Baldwin of Hainault, one of several claimants to Flanders, and promises another daughter Adela to Frederick, son of Frederick Hohenstaufen of Swabia, German anti-king. Increasingly under pressure in Flanders, Matilda conspires with William Clito, her first cousin, against Baldwin of Hainault, promising him Flanders and her daughter's hand. When her father suddenly dies, Matilda breaks her daughter's betrothal and marries William instead, together claiming Flanders, Normandy and England. Clito's French allies claim various parts of Normandy, also joining the Emperor Lothar in supporting Baldwin of Hainault against the uppity couple. The Pope once again rails against Matilda's marriage and is encouraged by Stephen of Blois, the French and others to forcibly separate the two. "
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A few quick thoughts

Charles would be too wrapped up in Flanders to really try anything in Denmark, unless his Danish cousins supported (like OTL) Lothar and he claimed Denmark as part of that whole conflict.