Eurail Global pass would be a rip off in your case, because point-to-point tickets would be much cheaper. Just take a look at some fares:
Istanbul to Bulgaria: Istanbul to Sofia ticket costs 20 euro, Istanbul to Plovdiv 16 euro. Train tickets within Bulgaria are also very cheap, so Eurail pass is useless there as well.
Bulgaria to Romania: Sofia to Bucharest standard fare is about 30 euro.
Bucharest to Brasov (?) is from 25 (app. 5,6 euro) to 50 RON (app. 11,2) depending on train.
Brasov to Belgrade would cost you about 25 euro, while
Belgrade to Budapest is just 15 euro.
Budapest to Bratislava is 17,5 (or even cheaper in case you ok traveling with change),
Bratislava to Vienna is 14 euro by train or half that by bus.
Vienna to Liechtenstein is perhaps the first leg in your entire itinerary where you should think about buying you ticket in advance, that way you can count on very good price, because discounted tickets start from only 29 euro and can be easily bought via
Austrian Railways (OeBB) website. From
Liechtenstein to Switzerland you can actually just walk on foot, but if you mean journeys like Vaduz-Zurich/other destination in Switzerland, then it's also possible to do it rather cheaply by means of taking bus to Swiss stations (Buchs or Sargans and then using SBB SuperSaver ticket, e.g. Sargans to Zurich costs 16 CHF). For
Switzerland to Slovenia you can get cheap spar ticket for the overnight train Zurich-Ljubljana (start from only 29 euro), although again makes sense to purchase this ticket in advance via
Swiss Railways (SBB) website, because such tickets are sold in limited number per departure.
Ljubljana to Zagreb is 16,4 euro and no need to buy in advance, it's standard price.
Zagreb to Sarajevo is about 30 euro. From
Bosnia to Serbia there are no trains currently, so you will have to use a bus or travel back to Eastern Croatia in order to catch train to Serbia (btw, you can also easily catch a bus from Sarajevo to Croatian/Montenegro seacoast, thus avoiding second journey to Serbia at all).
Serbia to Kosovo: again no rail connection, only bus. Same goes for
Kosovo to Montenegro and
Montenegro to Albania. You can only count on trains within Albania.
Albania to Istanbul is tricky, you can either travel via Macedonia & Bulgaria (with train on some parts in Macedonia & Bulgaria) or travel via Greece (with domestic train there, but no rail connection across the borders).
So, as you can see yourself, most of the point-to-point tickets aren't very expensive and total sum would be far less than the one you would pay for 2 month global pass. And the best thing is that basically there is just 2-3 legs (unless you plan extensive travels within Switzerland) where you need to think about buying your train tickets, while in the rest of the cases you can simply buy your tickets before departure.