Well, considering you have a JAMMA cabinet, you can plug and play TONS of boards. The question is, what do you want to play, and can you deal with it with an MK layout? Some games (like Double Dragon, for example) use three buttons to play, and you can deal with the layout to play that game. It's actually not bad. Probably the same thing for Blitz as well. Some games (like the aforementioned Killer Instinct) you'd be better off to pick up another cabinet, due to the fact that KI uses 6-buttons, Street Fighter layout style.
The first thing I'd do is figure out what games you are interested to play, and then go about how you are going to get there. If 4-player is your (and family/friends) style, then I'd suggest you pick up a 4-player cabinet. Blitz, NBA Jam, X-Men, etc. Any of those would get you three button capabilities, which is what MOST games used. Obvious exceptions are MK, Street Fighters, Killer Instinct, Dungeons & Dragons, Tekken, and games of that nature. Keep in mind, though, a 4-player Turtles or Simpsons only uses 2 buttons, so beware of those cabinets if you're looking to play different games.
If the fighting games are your thing, then a Mortal Kombat cabinet with the run button drilled is ideal, as you can play all 5 MK's on it (MK 1,2,3,Ultimate, and 4).
The second fighting-style cabinet you'd want to pick up would most likely be a Dynamo cut-corner cab, as 99% of these were originally Street Fighters in some form or another. These cabinets give you a generous 25" monitor, and 2-player 6-button layout. This will allow you to play (in addition to many standard JAMMA games) any of the Street Fighter incarnations, Tekken (with the extra kick harness), Dark Stalkers, Marvel vs. Capcom series, etc. Also, the benefit of this style cabinet is that it was built to have the monitor rotated, so as you get more familiar with arcade machines if you decided you want to play vertical games for a while (a multi-classic board, vertical airplane shmups (shoot 'em ups), or the like) you can do this in that cabinet. It takes about 15 minutes, once you're familiar with the process, and is rather convenient compared to other cabinet types.
Hope this puts you on the right track to what you're looking for!