Gotta think the massive attendance bump in Zagreb puts them on the KHL's radar.
Its highly unlikely Medveščak could join the KHL, though their attendance is good for Dom Sportova, that huge spike comes mostly from their Ice Fever event when they play out of Arena Zagreb which they sell out the 4 matches. Damir Gojanovic has shown interest in the KHL and said it would cost them 6-7 million euros per year but according to most people, they would need around 10 to be able to do anything. If they are able to somehow pay the rent to use Arena Zagreb the entire year, their roster would need to be almost all imports as Croatian hockey is simply too weak at the moment to take on KHL clubs.
Remember, just about two years ago they were playing in the Slovene Championship, drawing no more then 1000 people and with that roster would be beaten badly by any club in the EBEL. Medveščak is better off growing hockey outside Zagreb, improving their junior system and then maybe try KHL, but for now, the EBEL should be just good enough.
Based on all rumours, the KHL has zero interest in Croatia, they want the top Austrian clubs (highly unlikely), a Slovakian club (set in stone basically) and looking at clubs from Germany and Italy. (Germany most likely, Italy...somebody rich must be backing this club)