I installed LED flashers in all locations, it really makes the colors pop. WARNING - they are REALLY bright and it takes some getting used to, especially the backboard flashers. Strongly recommend a red flex head flasher behind the Fury target, that is a must even if you don't like them elsewhere. Also, some blue flex head flashers under the Tessaract looks great too.

A $100 Polk PSW-10 under the machine makes a huge sonic improvement. They could be had for as little a $50-70 for a while but the best deal is $99 shipped on ebay from Polk direct.

In addition to what TheFamilyArcade mentioned, Hook makes a new sling plastic that allows you to see all the LOKI inserts on the inlanes. ebay.com link: New Stern pinball Avengers LE sling shot plastic mod See Clearly must have

I would get the Cube that includes the flasher that mounts behind the cube. The flashing effect is really cool as the cube spins.