Everything goes as per OTL until 1886 when Ferdinand fails to return and the pro-Russian government stays in place. Russians nominate Prince Nikola of Montenegro to be the new Bulgarian monarch, and Austria flips its shit hard. Eventually they settle on Peter Karadjordjevic (OTL's Peter I of Serbia) as the compromise candidate. (This could work since Peter Karadjordjevic was Nikola's son-in-law, and both the Russians and the Austrians believed him to be in their own camp.)
Changes? Relations between Bulgaria and Russia aren't severed like in OTL. Karadjordjevic was more democratic-minded then Prince Ferdinand (and Stambolov); and more importantly, he'd have recognition from Russia and most other powers. Which means slightly more political stability for Bulgaria, but only slightly.
With Russia's support, Peter would be able to secure some gains for Bulgarian cultural institutions in Macedonia, but he'd be reluctant to suppress Serbian claims. So the cultural struggle for Macedonia would look more like Bulgaria and Serbia ganging up on Greece.
There will be no military actions against the Ottoman Empire before the 1900s. Not because Peter wouldn't want it, but because Russia wouldn't want it - and because creating a Balkan League between Obrenovic Serbia and Karadjordjevic Bulgaria would be...difficult.
The Obrenovic dynasty in Serbia will almost certainly fall in the 1890s. But the Karadjordjevic might have trouble succeeding them, because Austria is guaranteed to block it. So there's no telling who the next King of Serbia would be - though it would be darkly ironic if Ferdinand of Saxe-Coburg-Gotha winds up on the Serbian throne while Karadjordjevic sits on the Bulgarian throne...