Quick notes:
If you get the fill can, be sure to get the "fixed" version, as it doesn't cause errors with decal application. The current RTB version should do.
If you use JVS doors, do NOT download JVS_Content unless you wish to. A default replacement called "script_legacydoors" is available, as JVS_Content apparently had a crapton of problems.
Tophius, General, and many others have also created a huge n' wide variety of bricks that emulate LEGO bricks themselves, many of which can be found on RTB, so keep an eye out for 'em!
Bushido has created the massive "Tier+Tactical" add-on weapons and tools, and even a lil' set of auto-sentry guns. However, it's up to you if you want those or not. They are fairly ubiquitous and colorful as a set of guns, though.
Blockland currently has a default bot system, so there are some add-ons which might be helpful if you wanna do stuff with those, such as input events for when bots are damaged, etc. etc.
VCE (variable conditional events) remains a useful events set for pretty much anything.
There's a lot of stuff to consider, really. You should pick n' choose what you want, my suggestions are pretty lame.
