At a glance, MKX has the same tiers as Injustice. Bronze for your weaker characters, Silver for the mid-power characters, and powerful Gold characters. It would be easy to think it’s the same set-up. In fact it is quite a bit different.
For the most part, Bronze characters are now unnamed members of the various factions, such as Troopers and Sergeants for Spec Ops or Lin Kuei ninjas for the Lin Kuei. They lack a 3rd level special attack, another change from Injustice, but are just as weak as you’d expect. This avoids problems with people seeing their favorite characters lumped into the least powerful category and I think the change works for the most part.
The most welcome change is the silver tier which is where the primary versions of the major characters reside. Silver has become the standard and, unlike Injustice, the game is entirely playable with nothing but silver characters. Assuming your levels and fusion is high enough you can easily beat the final tower in the game with a team of all silvers. Another change is the 3rd level special attacks are no longer block breaking finishers but simply more powerful attacks that usually carry an added bonus like increase to defense for a few seconds or increased critical chance.
The Gold tier are specialized version of the silver characters with a few notable exceptions like Kitana or Quan Chi. They effectively mirror their Injustice counterparts with the replacement of supermoves with X-Ray attacks that effectively have the same result. Unfortunately, Gold characters are mostly earned by using the new premium currency in the game, but it appears quite a few gold characters pop up in the Kard Pack on rare occasion.
While I do not care for the new premium currency or how Gold characters are now purchased with that currency, I will say that I appreciate how the game allows silver characters to be competitive as a trade-off.