There are MK7 threads and this one as well: http://mhwiki.forumotion.net/t6446-3ds-friends-code-and-information-section
As far as online games most of those are on the way but like stated by RDC RE:Rev has a pretty good online but still limited. Here is a list of the ones that are out and coming out that I think will have the best online and worth getting.
Dead or Alive/Street fighter: both fighting games and both appealing to look at and play if you are into fighting games
Kid Icarus, it will have multiple local and online multiplayer features and looks great.
Heroes of Ruin is a hack n'Slash from Nspace with full voice chat and drop in/ drop out multiplayer. Though it is basically a Diablo clone it will have daily challenges and company supposrt to keep the gameplay and multiplayer aspects constantly fresh and worth playing.
Super Smash Bros: Not much information on it other that that it will have online...
Animal Crossing will have a multiplaye function as well that will actually allow people to play games and do stuff together..
There are a few DL games from the eshop with online multiplayer as well but not sure which ones. I know there are more games than that out and coming out for it with online but not sure beyond the one's I've said as those are really the only ones I've looked into. I know HoR is suppose to be the one with the most advanced and most online features followe by Kid Ic. that I know of.
And not really a game or Multiplaye but there is Swapnote/Letterbox as well. Which can be fun.