Boss battles could be fun, if done right.
It would take a lot of time and effort though. Considering actual player models, arenas, weapons, bug smashing / balancing.
But, I think that is part of the reason for servers with minimal quality. Hosters may be working on their servers on their own, or with a handful of people. Their interest levels will most likely decline soon if they don't have a finished project within at least a month or less. Though this can be enough time for a decent server, we need to consider time constraints, writers "builders" block, and lack of motivation (though this can be fatal to a server).
If someone just simply dedicated himself and a "staff" to simply take one idea and slowly add-on to it over half a year or a full year, you may end up with a server with great gameplay / quality. The thing is just staying motivated...