Well, I came here with background mostly in UEd 2.x for Deadly Shadows and UE 3 editing (UT3, Gears of war PC), so you can imagine my frustration when I tried to use DR for the first time. It felt so backwards in comparison to what I had at my disposal (I mean UE3, not Deadly Shadows ). Like, how can you have no GUI material editor in this day and age... I needed a whole weekend to get the idea behind the idtech4 and to get used to the controls. I changed almost all shortcuts, tweaked the color scheme too. I wasted at least a few days of work to get the basics of writing materials and importing models from 3dsmax. I did a sort of "tech evaluation" to see what I'll be able to make on my own, and boy that was long and painful. I'm more of an art person, while idtech4 definitely needs working in pairs: art + programmer duo can make the most of it in efficient manner.
I don't know your background, but from my standpoint, the thing you're trying to make is way too ambitious. I never released a TDM FM, but I have 2 (mediocre) TDS FMS and several tech demos under my belt. At this point, I'm better at making textures, models and environments than gameplay, so I'm trying to make some standard gameplay and small map, but with 100% custom textures and models. You're trying to make all that too + unique gameplay, + large environment. You're setting the bar very, very high, and IMO that can be very destructive, especially when you're a perfectionist. Noone wants you to burn out, we'd sure love to see a finished map
(Btw. I don't understand why the wet dream of every Thief(-ish) community mapper and their mother is to have a large explorable city in one map. There's a whole series of games that has already proven that it's no holy grail, more like boring boring – I'm talking about AssCreed series)
Anyway, I'd probably cut on my goals at this point, or go over all the references, design docs etc., to see what's doable now, i.e. within a few months. What I'd like to do, what's important to me. Definitely not something that huge. Maybe you're that good, and you are able to do this, but to me it sounds a bit like one-man MMO project – noone has ever finished any