Valky Of the ones on your 'left to play'-list, "Endless Rain" and "Chalice of Souls" are also sprawling cityscapes like "Between These Dark Walls" and "Shadow Politics", meaning two solid 3.5-missions for you. "Lord Robert's Painting" also falls into this category, though to a lesser degree. "Prowler in the Dark" is one of
Melan 's works, you really can't go wrong playing his other FMs, though most (all?) of them are for Thief 2 and/or The Dark Mod.
Yandros's recommendations are all solid, though some of them are a little off the beaten path. "Sepulchre" is a more ambitious "Bonehoard" mission, and "Returning a Favor" is a nice whodunnit. "Lorgan's Web" takes place in the Discworld universe and "Cult of the Resurrection" is a little old, but solid fun nevertheless.
A couple things to keep in mind... several good/classic T1 FMs got later ported into T2 and are most likely better played there. Second, because the opera house mission in T1 came in an expansion there really aren't that many FMs like it for T1, though the concept of secret passages is prevalent in many FMs. You will have better luck in the T2 FMs.
A few more T1 recommendations from me:
"Gems of Provenance" - a 4-mission FM extravaganza. The third mission "City Public Water Works" is a veritable masterpiece.
"Shunned" - another off-the-beaten-path FM, this short adventure focuses on horror. Worth a mention, at least.
"Project Hammer" - this mansion heist FM got recently updated and presents a challenge at the start, but is otherwise good fun.
"Nigel's Hidden Treasure" - To be honest I don't remember much about this one, save to say that I enjoyed it.