Since Linux has no concept of a base system, it's a stand-alone kernel with a hodgepodge of crap around it - this distinction makes no sense on Linux.
/opt is generally for software distros for which you don't have source; only binaries. Like commercial software packages. More common on Real UNIX(R) because most Linux users outside enterprise aren't running commercial software. You're putting your $500k EDA software under /opt.
Steam says hi.
On Windows, a common Steam library exists in Program Files directory, therefore not user specific. On Linux, each user has a separate Steam installation and library. I'm not sure why there isn't a common Steam library on Linux, but /opt would be a good place for it.