I didn't have any immediate ideas for any more G-Series geomorphs, so I decided to start on the F-Series: Floor Features. I'm not sure that any of this series will be spectacular, but they're more or less necessary.
I'll start with two geomorphs based around grates in the floor. F1 has several 10x10 areas where the floor is replaced by a square grate, allowing those above to look down on those in corridors below. These grates have large gaps that small or skinny characters might actually fall through, if they don't tread carefully. The grates may be used as vantage points to spot invaders or pour/drop substances down on them.
F2 has round grates covering circular pits. The gaps between the bars are a little smaller, so perhaps no one halfling-sized or larger will fall through, but it's possible to get a foot stuck if the characters aren't careful. One of the pits has an exit to a corridor, so if the adventurers can somehow remove or pass through the grate, this could become an access point to the lower level. Either kind of grate may be made of wood, bone, iron, or some other substance entirely.