The main problem (things that Markos may not even realize yet; because we are NOT talking about a leader that is extremely bright--at least when it comes to keeping his fellow Travelers together, and ONLY gets his people to follow him through sure violence and threats...as he REALLY only cares for himself, as we saw he will kill even his own people to accomplish his goals--a true sociopath). But without spirit magic in the world, some major problems that aren't immediately seen as issues may quickly become HUGE problems--problems that even Markos must deal with as this issue will manifest first IN Mystic Falls were the barrier between the "Other-Side" (or whatever it will be, more than likely the "Dark Dimension") is now the thinnest--allowing the little "dark spirits" who are more than likely Phantoms--to start terrorizing all the witches that they can (possibly beyond this if they change up their approach, which I think they will to some degree). It will be comparative to what happened to Bonnie when she tried to use Dark Magic and some type of "being" tried to attack or even possibly take her--these beings may have all along been the Phantoms.
Markos may get part of what he wished for (since the spell was activated for quite a long time--it may be complete within Mystic Falls and the outlying areas), but he may have NEVER guessed that something FAR greater than his entire group is now hanging around...all thanks to his spell. AND, it ISN'T interested in thanking him, just "nullifying the world. So, for awhile, Markos, may even team up with the Scooby Gang to handle this threat (till things seem like smooth sailing, then I imagine either Markos will try to take the Scooby Gang out quietly, or Damon will beat him to the punch and kill him in a very and "run of the mill" process that may even make them think that he DID die of illness, etc...
It'll be interesting to see who the final "big boss" is: Markos, Phantom(s), or even a kitsune (though doubtful on that, but STILL a possibility. I just hope the writing improves. It has been terrible lately. I really DO think the best went onward to the TO...