yourworstnightmare
Very likely that Greece stays out and Bulgaria never joins. They want land from both Greece and Serbia but are unlikely to join the central powers in such an exposed position.
With Russia so much stronger, [no Caucaus front and able to trade through the straits], Britain stronger because no need to fight the Ottomans and Serbia also stronger the two central powers are very exposed. Could see allied forces operating in the Balkans in support of Serbia and Italy joining the allies earlier. This puts Austria especially under huge pressure.
At least, likely to take a year off the war and save a lot of lives. At best once Turkey joins the allies and aid starts reaching Serbia, removing the change of an early victory there the central powers make terms.
Possibly a bit difficult to get the Ottomans in the alliance but if Enva Pasha is isolated there are good chances of keeping it neutral, which would be nearly as big a gain.
Steve