How's this one,
@Skallagrim : the Netherlands are left to the Holy Roman Empire instead of Spain.
One I like, obviously; being Dutch and being also a great fan of the Holy Roman Empire.
You can spin it in multiple ways, and there are realistic scenarios where things still turn out quite the same way. Different particulars, but the same general sort of outcome. Dutch notables were already irritated by the centralising reforms of Charles V, after all. These objections will not suddenly fade away.
However-- let's instead look at the (no less credible!) possibility that this altered inheritance changes things significantly. If the Low Countries go to Ferdinand in 1556, then the appointment of Margaretha of Parma as governess of the Netherlands would be avoided, and that would certainly be a good thing for relations between the sovereign and the Dutch. The whole dispute between the Dutch notables and Margaretha's chief advisor, Cardinal Granvelle, would be averted. This would therefore butterly away the foundations for the pact of rebellious nobles who would ultimately secede their lands from the Spanish crown in OTL.
Furthermore, Ferdinand I was more flexible than his predecessor, and he was carefil and cautious in all diplomatic efforts. This implies that if the Dutch nobles and notables refrain from Protestant trouble-making and the Dutch economy keeps turning a profit, Ferdinand will be hands-off and allow the Dutch to generally govern themselves.
This, I think, should suffice to prevent the Duch revolt. The Netherlands will remain a united league of seventeen provinces, governed from Brussels. It will remain tolerant of Protestants (something Ferdinand I did not see as a major issue), but its institutions will -- for the time being -- remain Catholic. There will be less of a clear line between Protestant and Catholic provinces. The North-East of this united realm, in particular, will see more of a diffusion.
...I rather think that, for all that it lacks the Netherlands, Spain will be better off without having to fight a lengthy war over the matter. The Holy Roman Empire may be bolstered as well. In particular, I suspect that the "Dutch solution" to the religious divide may prove attractive in various regions, and cause these regions to become functionally neutral in the great disputes over the matter.
This may in itself have several outcomes:
1) When a counterpart to the Thirty Years' War is set to erupt, the "neutral"/"mixed" states seek to get uninvolved, but the Protestants will not desist and cause unbearable tensions. This leads the neutrals to side with the Emperor to quell the unrest. Presumably, the deal will be that any Protestant who sides with "the rightful authorities" will be guaranteed the freedom to have his own religion. This leads to a much shorter war, which the Protestant side loses badly. The effect is that "moderate" Protestantism becomes accepted within the HRE, but the more militant strains (and sepecially local rulers who would use it to defy the Emperor) are ruthlessly stamped out.
2) The "neutral"/"mixed" states effectively block both sides from fighting each other on a large scale, since whichever one transgresses on the middle ground will find those states suddenly aligned with his enemy. The effect of this allows the Protestant states to
de facto secede from the HRE; an outcome that the Habsburgs will long refuse to recognise, but which they cannot reverse. War is averted. The HRE is reduced in size, but is now home only to a majority of Catholics and a minority of "moderate"/"loyal" Protestants who are secure in their religious rights.
3) The Emperor doesn't accept that the "moderates" are standing in the way of his ambitions of quelling the insurrections by force, and pushes through with military might. The moderates all flock to the Protestant side. This is the closest situation to OTL, but I think that because of the political backidrop, you msy well see sizable forces of Catholics fighting
against the Emperor. I think that this, too, may result in a shorter war-- with a more decisive outcome. The HRE will be split between a Catholic-Imperial South, and Northern leage of states that is mostlt Protestant, but also includes Catholics; all united in opposing an Emperor who has... overstepped.