When we played Rolemaster the companions were a bit overwhelming with all of the options, so we had to pare them down. Same with GURPS. This was my first introduction to Levers in designing a campaign. There are some systems that are ready built, and anything you do to them are outside those parameters are homebrew rules, and not necessarily well received or expected by the players. I like those well enough. But the toolkit RPGs GURPS, Fate (and FUDGE), Rolemaster- those were my bread and butter, being able to tweak little things by using optional rules that were included. Those made me prepared for using engines to design hacks- Powered by the Apocalypse, Forged in the Dark, GUMSHOE, Rooted in Trophy- those systems that are released to be able to adapted.