The main map I'm editing right now is C6M1, Prague. Visual changes so far include making ruined buildings seem intact:


I had to go the "patchwork" route, taking and manipulating existing models from the map in order to do this. The building in the first picture, for instance, has 4 roofs (because the base one was the only one with the exact length, missing space had to be covered). What can't be deleted - debris, ladders, etc. - is pushed below the ground level.
Hex editing proved useful up to a point. Multi-part models, probably because they are landscaped first and made into a scene in the editor later, have proven a tough nut to crack (simpler objects have so far been much more malleable). I also mistakenly thought it was possible to duplicate models from other missions, but H&DD seems to have tables that dictate what can be used in a level; from my understanding, if a model is not on the list, it can't be called into the scene (I've yet to find out if this is level-specific or a general list).
Not shown in the pictures are also other changes like no lit up street lamps (the mission is meant to take place during day), tables and benches outside the restaurant and a depot/garage of sorts. I'll post these as soon as possible, and if there's any interest in them as well.
So, for now the main point of this post is to just condense some notes about the mod. So far I'm only working on Prague's first level. The modified mission is meant to be based on a Czech/SOE WW2 joint operation which involved the public assassination of a high-ranking german officer, with some edits. The plan is to have the officer arrive in the level on an open-topped vehicule, assisted only by his driver. German patrols are present. People go about their business. Players will have a time limit before the officer appears, during which they can opt to secure a disguise. Civilian clothing will be easier to get but provide less stealth. Another officer on the map may be killed but getting to him will be trickier. The mission is to make the attack on the high-ranking officer public, and to be quick about it. The officer must reach a certain checkpoint, and this is where the attack must occur.
In theory, civilian get-up would also allow players to recruit some nearby resistance fighters (more back-up), while the german outfit would allow players to requisition a truck in order to block the car's exit through the bridge. While I'm still not savvy enough with the editor, I'm not sure how this might be implemented but I'm willing to learn and keep trying.
That's it for now. I really am trying hard to do this, so hopefully motivation won't fizzle and I'll keep at it.