Territory size is something I'd like to see as a factor as well, I'd say it'd be best if it allowed for more upgrades(if that's on the table of course) but they are more expensive. It only makes sense. Let's say one Defensive Upgrade option is Heavy Stubber Emplacements. Well if you have a bigger territory then that means there's going to be more gun emplacements, hence it should cost more. Perhaps for successive upgrades it should be proportional to the cost of the first upgrade, such as the initial cost +50%, something simple, plus naturally since it's bigger it's naturally going to have a larger effect on the territory. Also keeping it simple would be a sort of modular system, small Territories could have for example 1-2 slots for an upgrade, but each upgrade could have Tiers to upgrade your upgrade, while larger ones can have more like 5 upgrades. The numbers I'm using are kinda arbitrary, but also I think that would probably work well. Small has 1 upgrade, medium 3, large 5, and each upgrade can be improved say 3 times.
I agree that what I'm calling "No Man's Land" missions should be randomized, pretty much exactly like Mordhiem. That would represent exploring the Underhive in search of opportunities. You get to choose, but from what is available. Something else that might be harder to pull off is there can be a random chance that one of the missions you pick is actually an ambush, and your goal would be to fight your way out(lower chance as it'd be annoying if it happened too often). This also means that you can ambush someone else, which is easier to represent as you just pick it.
Defense/Offense I agree with, your own territory shouldn't be safe. However I'm not sure how Defense should work. It'd be too easy if your whole gang gets to be there right away, especially with the upgrade system, and Necromunda did have a pretty good reinforcement system for a dice game, this I think should be represented. You have a smaller amount of gangers there at the start, but other gangers respond. I think it should be tied with assigning Gangers to work a territory, the ones you assign are the "garrison" and need to try to hold off an assault until help arrives. This could also be tied into upgrades and prep missions, as then you can upgrade reinforcement times and also sabotage them to give yourself more time.
What I think would work well for a "No Loot" outcome would be secondary victory conditions. Like say you're assaulting an enemy water tank. If you can take over area you can loot or capture it for additional resources, or if you know you can't win the fight then there's secondary raid objectives, like targeting it and destroying it instead. There shouldn't be no opportunity to get loot, but maybe just victory conditions that don't give a loot bonus, with a bonus loot reward for a more favorable victory outcome. To me I think if you drive off the enemy there should be loot, because you have time then, but as I was saying there could be objective victories that if you complete and withdrawal you only get whatever you grabbed during the fight.
Gang size is tricky, I definitely agree that it should be bigger than Mordheim. but the magic number cap is hard to say. Too big or too small both would have issues both mechanically and lore wise. I think it should be tied to territory in some way, maybe settlements, or maybe a unique territory type like Gang Hideouts. There could be a gang rating system like Warband rating that dictates how many Settlements you can manage. I'm spitballing here mostly, but writing it out I think it should be a mixture of infrastructure and rating because that would add depth. As a starting gang you have to grow your reputation/rating to control more territory, however since rating is most likely going to like xp where you don't lose it, there should be a way to lose capacity. Fortunes change, so the reason I present it this way is that your gang should be able to both rise and fall, the best way I can think to do this is you start losing territory you lose capacity, maybe have the lowest ranked gangers abandon you if you can't support them, or have a Loyalty system the lowest loyalty gangers leave first if you lose your capacity for too long, and your most loyal stay with you.
Yeah it'd be great to get some other opinions, more theorycrafting would undoubtedly get fresh perspectives or at least a consensus of more people would have more weight for the devs to consider if they take a look.