The closest thing I've seen is the Superpower series of games. After playing one of them, you'll realize why more people haven't tried to make something like this -- the scope is so large and unstructured that many features are underdeveloped and the game as a whole is extremely unbalanced.
I think the gold, items, and experience are the most vital mechanics. To me, these are really what separates a MOBA from a shooter.
The real focus of 80% of the game is laning, getting creep kills, even getting hero kills to earn yourself more gold and experience. This gold and experience is then used to give you an advantage late game. If these don't exist, you need some other reason to force the player to do anything. ie: it sounds like the focus is to kill the enemy heroes - why should they do that if they're not going to get any gold for it?
If you want to strip off everything thats un necessary its not even necessary to keep the nexus destroying objective and the lanes. you can do a MOBA thats all about death match with no minions and no structures
These two conflicting views seem to capture something very central to the problem: Can the combat mechanics stand on their own and still retain the MOBA 'feel', or are the objectives of farming gold/exp and killing structures necessary to imbue the combat with meaning?
I'm starting to think that the middleground is worth exploring; I'm currently thinking of implementing destroying the two nexuses as the goal, but not utilizing any snowballing mechanics (e.g. gold and exp) or minions. Thus the only point of killing other players will be to temporarily take them out of the game.
Do you think this concept will make a simple MOBA or simply devolve into a deathmatch shooter with weird combat?
I would recommend that you add more skills, and that the skills are akin to ones in MOBAs. Also, there is normally at least some sort of snowball mechanic. The other things can be added later if you decide to develop it seriously.
Sounds like a good plan! I've started playing around with adding MOBA-ish skills (currently have something akin to Rammus's kit from LoL) and noticed something interesting: It seems like limited ability usage adds the most to the MOBA feel. Whether through cooldowns or 'mana', forcing the player to decide when to use their skills now appears to be a big part of the MOBA experience.
I'm still on the fence about snowballing, since blood champions (http://www.bloodlinechampions.com/) seems to have a strong MOBA vibe without it. However, perhaps that lack of snowballing is what causes so many of my experiences with that game turn into giant clusterfights; without it I guess there isn't a reason to hang back and only attack strategically.