The power of Forge is that you can change many things but only if you know the "secret inner workings" that aren't documented very well. Which leads to the next logical question is, "Why don't you write better documentation?" And truthfully I have spent a few hours writing docs but it is hard to understand the changes because Forge is constantly changing.
Someone posted a question on the forum and said that he was tired of always drafting the same cards and wanted to know how to change the draft cardpool (the cards that Forge uses to create the booster packs). Awhile ago I wrote a "set editor" that allows you do that but the problem is that the set editor was created before there was a separate set of files for draft. Yes the set editor can edit the draft cardpool but you have to move the files yourself.
In the lastest version of Forge, each card now has its rarity set in the "cards.txt" file. (Thanks to whoever added the rarity and http picture link for each card.) The good news is that the new rarity includes mythic rares. The bad news is that I don't know how Forge uses this new information.
So while Forge is moving in the right direction beware of the occasional blind turn. (Extending a metaphor is hard work.) Forge is always improving even though it is a work-in-progress.
If you want to change your draft files, unzip this file in your /res/draft/ directory. This way each card has an equal chance of showing up, since all three files have the same cards: common.txt, uncommon.txt, rare.txt.
Forge uses the files common.txt, uncommon.txt and rare.txt to create booster packs. Forge has 3 sets of these files for: sealed deck, quest, and draft. The sealed deck files are in /res/ directory. The quest files are in the /res/quest/ directory and the draft files are in /res/draft/ directory.
When you run the set editor it asks, "Do you want to edit the quest booster files?" which edits the files for quest. If you select no, you are editing the files for sealed deck. So you cannot directly edit the draft cardpool but you can move the files: common.txt, uncommon.txt, and rare.txt to the /res/draft/ directory. (For fun, cut and paste all of the cards into one big file and make 2 copies of that file. Then name all three files: common.txt, uncommon.txt, and rare.txt.)
You can download the set editor here.