Semi-permanent I don't know about. To me, that's my CG Outlaw (because it will eventually rust away like anything metal).
As for permanent, there are a lot of plans out there but here are some of the features I like:
1) Firebrick for any surfaces that come into direct contact with coals or flame. (It also adds thermal mass as it's denser than cinder block, and it's not difficult to clean.)
2) An adjustable charcoal/wood grill like the Grillery
3) A hood or closeable doors for the grill, and a rotisserie attachment
4) A separate smoking chamber that could double as a wood-fired oven
I've been looking over several pit plans over the past 5 years and I've still gotr more research to do.