First off, the Fourth Crusade was in the making for over a century. Constantinople had pissed off Venice one too many times. The fact the Roman Emperor didn't do anything about a massive crusader army right outside his city goes to show his stupidity. If it didn't happen, however, the world would not be the same. Fact is, no Fourth Crusade, you have Anatolia reconquered and a very happy Roman Empire. The balkan problems that existed post-1204 exist to this day because of that date. Before, there were no unity problems.
On to Tamerlane... Tamerlane could have done the job, but didn't. He shattered the Turkish nations, destroyed their army, killed a Sultan even, but never consolidated and never took further steps to bitchify his freshly won states. In the end, they all came back stronger than before, and the Romans were finished.
On to the whole Rabid West Goes Postal on Byzantium event, I don't see how this could happen. I mean, sure, if Byzantium went and sacked Rome a few times sure. But no Emperor would be that stupid. More likely is that a few would find themselves quite snug within the Roman sphere of influence, and a most others would be indifferent. Austria would be the largest problem for a resurgent Roman Empire, but even they would be smarter than to challenge Constantinople 1 on 1.
What I see would happen:
Italy would be a bit tense over it, but wouldn't torque their panties. Venice + Genoa were pretty nice to the Romans in their last days, and while the Romans might not be gracious enough to return the favor, they know better than to try and pull another 4th Crusade. Even the Pope himself almost went cactus on the Doge over that one.
France is too busy with England to care. England, likewise, is too busy with France to care.
After Spain appears, they are off in the new world bringing glory to Catholicism and Spain.
Austria, wary of the resurgent Orthodox power, would form a power bloc with Hungary + Bohemia and not pull any crap. More likely that Constantinople would make the first move, since it would be seen as more than evil for the Western Emperor to declare war on the Eastern. Of course, the Eastern Emperor has no such graces towards Roman "unity", nor does he have the liability of hundreds of minor states to keep content to remain on his throne.
Poland wouldn't mind having the Romans back, since it keeps her southern border relatively quiet. An alliance of some sort isn't out of the question either, if for no other reason to counter the Austrian power bloc. Lithuania, long the vassal of Poland, would follow.
The vast expanse of the Russian plains would be happy to see their religion beating up infidels, so while the leaders of the states may not like Constantinople's Emperor, they couldn't afford not to support him. General discontent and revolts aren't on any leader's Top 10 'I Love' list.
Now if you still want to have an alliance against the Roman Empire, look to the east. The Islamic countries would hate the fact that their long-time smack-toy is now smacking them around, and would do some nasty things to the Romans. Persia never held any love for the Emperors in Constantinople, and nor did the Mameluks, Arabians, or Iraqis. They would all love to kick her rear, but the Shiite - Sunni relations would mean two alliances, and a war between the two over who has the right to kill the Romans.
So either way you look at it, you have more than one group of nations divided amongst themselves on what to do. Ryoken or whatever his name is is just whining because we get more attention than his precious French EEP. Funny thing is, last time I checked, there was no civil war in the EEP... Hmm...
