...supposed to be a participation thread. ;)
Just go ahead and think of all the possible things Doc Brown could invent. No need for a theme here - you might think of inventions for mechanics of the game, inventions Doc & Marty could have really used in the movies or just hilarious ideas only crackpot inventor Doc Emmet Brown could actually put together. No need for drawings, but if you feel the need to draw the crazy inventions of others, PM and go ahead!
Here's my first contribution:
Enables Doc & Marty to speak to each other even if they are in different time periods! This way, they can always stay in contact. With only a few modifications, the Time Walkie-Talkie can also be used as a scanner to find people "lost in time" by tracking this device.