A 23MB patch for Marvel vs. Capcom 3: Fate of Two Worlds has set the foundations for "Event Mode", but it's also made some behind-the-scenes tweaks to the game's balance.
Very little is known about Event Mode at the time of writing, although Game Console understands that the first "event" offered in this free game mode will release very shortly.
Regarding the balance tweaks, the major character to be affected is Sentinel: a character many claim is overpowered, and who's subsequently become an online favourite. Sentinel has experienced a significant health-bar decrease, from 1.300,000 down to 905,000.
Also fixed in this patch is a glitch that allowed Spencer and Haggar to temporarily freeze a character to the ground. Akuma's infinite combo has also been removed.
Here's a brief video that explains the balance changes in more detail.