The goal of this deck is to start off with Training Grounds and start the Pack Rat factory. If all of my Pack Rats are swept or bounced, I have a backup plan.
Instead of discarding useless junk (lands, etc.) I will be discarding Reanimator pieces. Reanimating a dead Pack Rat or a Griselbrand to help me draw into a new one will help me recover from any sweepers, bounces, etc.
In addition to the main game plan, I have a lot of specific answers in my colors to certain decks in the Modern meta (Regress for a cheap Affinity hate, Hushwing Gryff for Splinter Twin combo), which can be used for their normal purpose, or discarded to fuel Pack Rat!
I feel that this build in particular will make Pack Rat highly playable in Modern, and might push it into a Tier deck status if done correctly.
P.S, If you would like to recommend a card, please check the maybeboard to be sure I haven't already considered it. If it's not there, feel free to comment! If it is, please post your argument as to why I should include it, and what I should include it over.