Well add to the mix the Dutch who will be worried it tries to swallow them up in some attempt to get all Germans under the banner.
Worried = "We shall try to play MegaGermany against Britain and France as neutrals instead of immediately joining German customs union", not "We immediately join the new anti-German coalition".
The Scandinavians (especially Denmark) for the big threat this presents. It could potentially even try and take over them for all they know.
Or they could try to appease Germany, like they did in say both World Wars.
The Ottomans- it may decide to try and liberate some Christians.
Theoretically possible, but then they would support Russia doing the same, and Russia is likey to do it far before Germany. Italy may be willing to do it too eventually, but it shall be years before it has the muscles to do it, even with German help.
Portugal- "If its enough to scare Britain then we're freaking scared"
True. At least until budding German economic hegemony gives them an alternative, but it shall be decades.
Spain- Spanish history of this period is not something I know but weren't they still a rather absolute monarchy? (sorry if not, they'd still find something to be scared about)
Weel, in this period they are tossing in the bed lika mad from semi-absolutist to semi-liberal and back again via coups and revolutions and various merry dynastic civil wars, all the while like grasping to the remnants of their colonial empire, I would say they could care less about the size Germany gets.
The Italians- Well, the Austrians are sitting on a large amount of Italy from the start, who's to say they may not want to add more?
Well, if a liberal German-Austrian Emperor is willing to unite Italy in an efficient confederation as part of the Mega-Empire, many liberal-national Italians may be easily willing to call him "My Emperor", as long as he gives Italians a fair power share.
There's this variant of the PoD I've proposed, where a liberal Habsburg takes the lead of the national movement in Germany and Italy and creates a confederation of Germany, Hungary, and Italy.
In 1848, Italians were willing to follow *any* prince that seemed decent and willing to unite Italy. Charles Albert, Pius IX, even a liberal Habsburg.
At the least they'd be scared of them upsetting the balance of power.
This is farcical. Since unification, Italian politics has not given a dime to concerns about the balance of power, they only care about joining the strongest side that can help them fulfill their irredentist, colonial, and would-be great power objectives, and shower them with the biggest economic/political/strategic advantages. The strongest their patrons the better. Unless MegaGermany is keeping a lot of irredent Italians under its brutal thumb, Italy is the power sure to join it enthusiastically from the start.
Overthrowing their allies' governments
When ? Where ? I just notice peaceful liberal revolutions happening. Britain did not join Holy Alliance's counterrevolutionary expeditions even at the wrost reactionary nadir of its hsitory in the 1815-1830 period, to say to do this now on their own is ASB.
and threatening the stability of the continent is as good a cassus beli as is needed. This isn't EU3.
And despite what armchair strategists may think, the British people are not mindless gung-ho robots ready to unleash naked military aggressions on every nation of Europe with size problem, at the "balance of power" battle cry. The public, press, and parliament demand something like an allied country being invaded before they put British blod and money on the line. This ain't a computer game, indeed, where UK AI starts flinging DoWs when any other country reaches a province threshold.
No doubt if MegaGermany forms by faultless means, someone in Whitehall may start looking for possible ways to cut it down to size, in tandem with France, if they not not too busy dealing with Imperial matters the other side of the globe (since Imperial concerns always take precedence on balance of power ones), but these things take time, opportunity, and political plausibility to set up.
And Britain stayed neutral in 1870 because it was none of their business. There was no big threatening mega power involved, just two middle-rate powers. To support either one would have messed up the balance more than just letting them kill each other.
Fine, in Alt-1870 they may join the fray on France's side IF France can provide a decent casus belli that the British public can accept to spill blood for and is not the obvious aggressor (NOT a given). Russia shall almsot surely be neutral, and leap on the opportunity to carve up some extra bit of Ottoman hide. France/Britain vs. Greater Germany/Hungary/Italy. A fascinating fight to have in a TL, no doubt, but my bets are all on the CP side. I'll bring popcorn. The Central European Confederation in Paris in how many months ?
First; What's your obsession with French greatness? The French were not great in this period. They were a second rate power.
With homicidal urges against anyone that threatened their would-be continental hegemony.
Second; I very much doubt such a lovely Germany would sign over the Ottomans to Russian conquest.
From a 19th Century liberal European PoV, Christian authoritarian rule is much better than Muslim authoritarian rule.
Third: You forget the big one; Poland. The arrangement they had with Prussia and Austria-Hungary here was a good one. They all kept the Poles oppressed and ruled happily, all this now is changing. The Russians are not going to be thinking of invading Turkey when the stability of what they currently have is under threat.
IF the Polish nationalists are restrained and foresighted enough to get on the good graces of liberal megaGermany instead of alienating it with bullheaded separatist riots in Posen and the like. Since these are Polish nationalists, I won't keep my breath waiting for that to happen.
If Britain and France tell Russia they're going to go in and fix Germany Russia will be wanting a part of that.
Hardly. 19th Russia got pretty much what it wanted in Central Europe t the Congress of Vienna. Afterwards, their geopolitical directions of expansion were wholly elsewhere, not in Germany. The Tsars were not Trotski or Stalin.