Recently, my Modern England class moved into the 17th Century and I've become fascinated with the story of Oliver Cromwell and the Commonwealth/Protectorate of England. The Anglo-Dutch Wars were largely skipped over in my textbook, so I got a brief summary of the events from Wikipedia. It seems like, although Cromwell hoped the Dutch might join the Commonwealth (as both states were Protestant and Republican), it was never a serious consideration. In fact the Dutch Stadtholder was sympathetic to the Royalist cause in the civil war, and outraged when they beheaded Charles I in 1649. Nevertheless, when the Stadtholder William II attempted to gain complete power two years before, the States-General (their equivalent to Parliament, I think?) turned to Cromwell for aid, suggesting the Netherlands might join the Commonwealth. They were probably not seriously considering this, but there may be some way, by altering events, to make the Commonwealth not only survive and flourish but even absorb/unite with the Dutch Republic?
-Perhaps if the Royalists lose the Civil War earlier, with Charles killed prior to 1649, the Commonwealth would be more free to send support to the Dutch States-General, putting the Model Army in Holland and later coercing them into unification.
-Going back even earlier, if Elizabeth I's army fares better when supporting the Dutch rebels in the 1590s perhaps the two become even more closely intertwined. Similarly, if Dutch religious ideas get a stronger foothold in England in the earlier 17th Century, perhaps the Church of England by the 1640s is more closely aligned with Dutch Calvinism and brings the two countries into closer sympathy.
-Perhaps if the Dutch Republic fares much worse in the latter stages of the 80 Years' War, it could drag on with the Spanish maintaning a foothold and continuing to threaten the Republic into the 1640s, further necessitating an alliance with England?
These are just ideas. If anyone else is familiar with this period, I'd be happy to hear more advice about how to make the idea work.