Shadowdark Finally Played Shadowdark

[Whizbang Dustyboots]

Probably not, but C&C reads a bit like TSR D&D but on a 3e d20 frame.

C&C also supports customization in the sense that you can probably just drop any 3E system on it and it'll be more or less fine.

 

[Whizbang Dustyboots]

I'm not in love with the SIEGE engine, but I've seen houserules that make it a little more straightforward.

Only the Troll Lords are in love with the SIEGE engine, and they seem determined to leave it as is forever.

 

[Shadowdweller00]

Back in the day, what passed for strategy was often hiring up dozens of apparently suicidal hirelings to be sent in to set off every trap and act as cannon fodder, neither of which I think are common today. (For one thing, DMs are much more likely to ask why these NPCs would agree to do this stuff.)

Hmm. Next idea for a gauntlet: The PCs are a swarm of low-level mooks promised fabulous treasures by a group of famous NPC adventurers if they just "go scout out some temple complex".

The complex is filled with nasty defenses that the NPCs aren't expecting any of the PCs to survive. At the center of the temple are indeed fabulous treasures... guarded by a bunch of overpowered beasties that the party SHOULD leave to the NPCs. But could sneak or run past, potentially while the NPCs are fighting then. Or they could lead the NPCs into deathtraps and maybe steal THEIR stuff if they figure out the racket in time.

Yeah, it's a cliché. It's just been a while for me.

 

[Whizbang Dustyboots]

The new Armory book adds weapons with new traits. Also there are battle maneuvers. So that could provide some customization. I'm also aware of a lot of classes added outside the core book (such as the Adventurers Backpack). Plus there's fan created content such as a psionics system.
Choosing things like your primary attributes, races, etc, give some customization. It's not a bad system. I'm not in love with the SIEGE engine, but I've seen houserules that make it a little more straightforward.

Just watched this video for Nimble, which seems like it might be worth a look for you: