I have heard that we can play solo/group in our own homelands and pvp in Cyrodil and that we cannot travel to the other factions areas. On the map I see there are islands out there. For those that have played the other Elder Scrolls games are those islands controled by the usual factions? Can we go there?

I don't think we know enough yet to determine specifics about what areas will be available. From what I gather, the starting zones for all three factions with all feed towards Cyrodiil. Meaning, the general current of Questing will draw you towards the center of the map. Some ZOM developers have stated, some areas won't be available at launch but, will be opened to players later on. Most likely in future patches and possibly expansions.

I was speaking to someone earlier and I was told that quite a lot of your races leveling zones will be blocked off for future expansions.
Like in Elsweyr, you can only visit the North-West and South parts of the map and not the rest.
Can anyone add on to this? I'm starting to worry a teeny bit.

To answer the question though; I guess to be fully sure we have to wait and see.

Daggerfall Covenant gets High Rock, Hammerfell & Cyrodiil

Ebonheart Pact gets Skyrim, Morrowind, Black Marsh & Cyrodiil

Aldmeri Dominion gets Summerset Isle, Valenwood, Elsweyr & Cyrodiil