Dutch is a Germanic language, when Germany was formed all the different Germanic sub-cultures joined, was there a plan to include the Dutch regions into the Empire?

Before 1871, all the states that joined to form the German Empire (except Austria) had all been members of first the Holy Roman Empire, and then the German Confederation. So they had a common political history.

The Netherlands (and Belgium), on the other hand, hadn't been in any sort of political framework with "Germany" in three hundred years. In that time, those nations (especially the Netherlands) had formed a sense of national identity that didn't identify as "German" at all. The language, too, had by then diverged so far from the "actual" German that it was by all intents and purposes a different language. Like Spanish and Italian, for instance.

In other words, nobody even considered the Netherlands as German in any sense of the word.

Another direction I'm interested in is in WWII there was the Anschluss, was any efforts/plans made to attempt a similar sort of thing for the Netherlands? I mean pre-invasion, a diplomatic/political effort to join the two countries.

No. Not only had the Netherlands (and Belgium) developed 70 more years of national identity by then, but the political class in those countries was very different too (the Netherlands had a Christian Democrat government under Colijn, Belgium had a Christian Democrat-Socialist government under Pierlot).

And, at least in Belgium, the fascist/nazi parties that did exist (such as Verdinaso) pleaded for a unified Dutch-speaking state, and not for "joining" the Reich.

Dutch is a Germanic language

This doesn't really mean what you think it means. English is also a Germanic language, part of the West Germanic languages as are also Dutch and German. The Scandinavian languages are North Germanic languages.

The Netherlands was part of the Holy Roman Empire until Charles V tried to stamp out the Protestant Reformation. This started an Eighty year long war. In 1579, 17 provinces in the Low Countries signed to Treaty of Utrecht. In 1581, the seven north western most of these provinces formed the Dutch Republic. A map published in London in 1618, shows the British seemed to think The Netherlands was an independent country, the Hapsburgs did not recognize that fact until 1648.

I don't think Bismarck intended to include the Netherlands into the German Empire.

The way for German Unity he chose was the "Kleindeutsche Lösung" (Smaller/Lesser German Solution), as opposed to the "Großdeutsche Lösung" (Greater German Solution.

These differ in the inclusion of Austria. The most important German states by the 19th century were Austria and Prussia. Within the HRE and the German Confederation (respectively before and after the Napoleonic Wars) they always fought over hegemony.

Austria was in favor of the Greater German Solution, as it would most likely be the hegemon; whereas Prussia preferred a Germany without Austria, to take the role of hegemon for itself (which is the way it went in the end).

After the German-Danish war of 1864, according to Danish historian Tom Buk-Swienty, King Christian IX sought to join the German Confederation in order to keep the territories he was bound to lose and instead have all of Denmark join the Confederation intact. Bismarck however refused, not wanting to dilute Prussias power inside the confederation

The German Confederation dissolved after the Austro-Prussia war (also know as the "German War") in 1866 and Prussia created the North German Confederation, which after the Franco-Prussia war of 1870 gave birth to the German Empire under Prussian leadership.

Bismarck didn't want Austria inside his empire, he didn't want Denmark to be part of his confederation; I believe the same applies to the Netherlands.

You have to make an important distinction between 'Germanic' and German. The German Empire was made out of German speaking territories, minus the Austrian part of Austria-Hungary.

German and Germanic are not the same thing. Germanic is vague term that was orginally used by the Romans to describe the tribes north of the Roman border. These Germanic people were made up of tribes like the Teutons, Goths and Suebians. In the lower regions of Northwestern Europe where the Rhine meets see, the country that would later become the Netherlands lived other Germanic tribes like the Frisii and the Batavi.

But to linearly trace a countries ancestry back to these ancient tribes would be wrong. Since the fall of the Roman Empire and the beginning of the migration period the tribes and ethnicities have become mixed and influenced by one another. The Netherlands have at one time been inhabited by Germanic trbies, and are now a Nation of Dutch people speaking a Germanic language, but this does not make them 'German' or 'Sub-Germanic'.

Other languages like Dutch make a distinction between 'Germanic' people and 'Germans'. The Germans are called 'Duitsers' while the ancient tribes are usually described as 'Germaans'.

For your question about the Anschluss. This is a rather complicated one but you have to understand that Austria, next to speaking the same language, has been part of the large, rather abstract, German political territory for hundreds of years. Whilst the Dutch been a sovereign state since the beginning of the 17th century.

Hitler of course has his own view of the Germanic 'nation', but there was never the idea that the Dutch were the same as Germans.

There was a great deal of friction and debate among the German occupation forces and the upper echelons of the Third Reich about the long-term trajectory of its occupation policies for Western Europe. Officially, the Third Reich classified non-Jewish Dutch as "Germanics" (Germanen), but not "German" (Deutsche). They considered Dutch to be a Germanic language and thus would over the long-term be subsumed into the Thousand Year Reich.

The problem was that the Third Reich had very little idea as to how to realistically reach this Germanized Europe. Much of its occupation policy was made had to fit into an ideological extemporization. The dysfunctional satrapies and overlapping organizational structure of the Nazi state added further confusion. One view, favored by the NSDAP Party officials and the Foreign Ministry was to encourage National Socialist parties in occupied countries. This meant encouraging preexisting fascist parties and promoting their members into the occupation apparatus. These mini-Fuehrers would help create a mass movement for Germanization over the long term. In the Netherlands, the fascist party Nationaal-Socialistische Beweging in Nederland (NSB) led by Anton Mussert and Cornelis van Geelkerken became one of the main organs for Dutch collaboration. The Third Reich encouraged the NSB to become more antisemitic (prewar it allowed Jews into its membership and Mussert modeled himself more after Mussolini than Hitler) and Germanized.

In contrast to the NSDAP, the SS and Himmler favored an approach of selective Germanization through membership within the SS. This meant that a future Dutch elite would come from the ranks of the Dutch Waffen-SS divisions. Himmler's rationale was that speaking German and fighting in the crusade against Bolshevism would create a Germanized elite that after the war would return and assume a position of leadership within the Netherlands.

Many of the Dutch within the NSB hated Himmler's plan and even though they were fascists, they were also staunch nationalists. Mussert complained "the higher SS leadership considers the Dutch people German. It is terrible. What will come of it?" Mussert tried to appeal to Hitler to create a "League of the Germanic Peoples" with Germany at its head and an enlarged Netherlands as its chief subordinate in the Low Countries. Hitler had very little interest in this project. In a December 1942 meeting with Mussert he made it clear that while the Netherlands would not be treated as a conquered country but would be part of the Greater German Reich. As your question averred to, he saw the precedent to this the Austrian Anschluss. The Netherlands would become future Reichsgaue, but that was for the distant future. In the meantime, Hitler reaffirmed Mussert's leadership of the NSB, but the German occupation forces still controlled the country and the NSB was reduced to being mere collaborators.

Sources

Foray, Jennifer L. "The 'Clean Wehrmacht' in the German-occupied Netherlands, 1940–5." Journal of Contemporary History, Vol. 45, No. 4 (OCTOBER 2010), pp. 768-787.

Kershaw, Ian. Hitler, 1936-45: Nemesis. New York: W.W. Norton, 2000.

Mazower, Mark. Hitler's Empire: How the Nazis Ruled Europe. New York: Penguin Press, 2008.

You already got lots of good answers to your initial question, so I just want to mention that were contrariwise plans made in the Netherlands to annex German territory after WW2. Some of these plans would have enlarged the Netherlands by almost 50% and included even cities like Cologne, only without the German population.

Couldn't even imagine the answers I've received to this question. Thank you everyone for your input. Definitely gone above and beyond any answer I was expecting haha. Love this sub!