Recode everything. Lol, ok First off, lots of stuff is reskinned from D1, using the exact same code. Tractor cannon is the exact same code as a phalanx shield push/captain blast. Sentinel super is the same code as D1 Aegis from VOG. Lots of bugs and glitches that were patched in yr2 and yr3 in D1 were in the D2 beta, that's only possible if the code was copied before it was patched. Servers are the exact same, netcode is exactly the same. Returning exotics have the same code. I could go on. Did you know IO is the exact same size and shape as Mars, and Titan is almost identical to the moon? It's the exact same engine, Tiger, as D1. They said they were using a new engine but used the same one and made guns easier to balance instead so they can just change guns n not have to ticket with deeper buried code like map nodes.