Ok, and then ? What would happen after the Greek Caliphate ? I guess they would try to convince Balkanics peoples to convert to Islam, or maybe Russia, as they did in OTL with Orthodoxism
Depend of the era.
Before 700, they wouldn't try to convert them. You see, the fiscal income came mainly from non-islamic peoples and the islamic rulers had little interest to convert massivly the vaniquished lands (at the noticable exception of some local elites).
Even the Umar Pact didn't managed to have a realy conversion movment (in fact, it just encouraged the Arab to treat the converted people as non-Muslims).
Probably you'll have an actual conversion policy only if a christian power, or many christian statelets threaten the Caliphate : then in order to limit the influence of these ones within the Islamic borders thanks to christian polulations, the power would be encline to promote, or force, conversions.
Furthermore, you have to see that Christianity would be still an important part of the identity of such peoples : remember that the countryside of Syria and Palestina was still mainly christian during the Crusades.
The first step to conversion is the arabisation of the slavic and non-greek peoples, just like egyptians, syrians or hispanians were OTL. It's almost mandatory to have an eventual conversion.
For the Greeks, it's different : as what happened in Persia, their converted elites could be powerful enough to promote a conversion that is not only dissociated with arabization but turned against Umayyads.
Finally, you're talking about Russia, etc...I don't thinks that a conquest of Russia or in fact all greek lands is a likely plausibility. Caliphate armies are not infinitly reproducable or expandable and controlling Anatolia and let's say Thracia, Bulgaria, Macedonia would be the best they could have at middle term.
In fact i could see the survivance of christian "rhomanioi" states in Albania, Epire or even Morea that would act much like Galicia, Asturias, Navarra did in Al-Andalus : tributaries states of land that is not worth of controlling but still useful to periodically raid.
What do you think of that ? How could western Europe convert to Islam ?
Quite difficult. Considering as Al-Andalus have almost no conversion power outside within its own borders (and even there, i would think that before the Almoravids, Christian represented still a majority).
And as the Romhanion never had a real influence on latin christianity...
Outside a conquest of the west that would be difficult to do, not to talk about lasting control...I don't see how it's possible directly after the conquest of Constantinople.
That said, if we have a greek-islamic caliphate, it could be done at the long term.