I don't quite know of any games that are similar to crossover Skyrim and DayZ, but;
Brick Force: crossover between a fighting FPS and a building game, MMO
is it free?: yes
does it have in game purchases?: yes if you want better items :(
does it have premium?: let's just say this game likes to squeeze out money
is it fun (imo)?: i used to play it for a bit before going back to blockland, was fun for it's time. but now it just feels like it's one of those games that likes to take a lot of money. it's like combat arms. hacked and not quite good.
Elsword: anime styled MMORPG, available on steam to play
is it free?: yes
does it have in game purchases?: yes, but i think it's like NX in maplestory, it's for custom non beneficial items. currency is K-Ching.
does it have premium?: last time i checked, no
is it fun?: i played it a lot with my friends, and it was great to be able to go through missions with my buds, and not have random matchups. i don't know, it's kind of a diff experience for other people