Edit: combined summary of thoughts:
It would be useful to be able to have plots in partial operation, ie. only produce some fraction of possible output, with the settler idle or perhaps assigned to guard duty while otherwise idle. A plot might be in partial production for any number of reasons, eg. not producing enough to exceed defence values, not consuming more power than is available, not having enough manpower available, ...
For tier 1 plots, it would be useful to be able to have settlers work part-time on multiple plots, as this would allow you to establish a new settlement with a full suite of facilities zoned (food, water, defence, housing, power), but have the settlement be self-sufficient with any number of settlers.
It would be useful for a plot to evaluate if it would cause a shortage if it were to upgrade as one of its upgrade conditions, and for this to be reported in a 'future needs met' meter for each resource type there is some dependency upon, in addition to the existing 'current needs met'.
I'd much prefer that settlers building stuff consumed resources, rather than using the sensors as a resource stand-in; this would both delay expansion as well as not result in a late-game glut of resources that would usually have been spent on building settlements. On easy mode, I think that I'd cost it as n units of resources, and consume whichever resource has the most in stock at present, on more difficult settings it should require the same resources the PC would have needed (perhaps pull costs from Homemaker as one of the mods that allows you to build nearly anything). Ideally there would be resource-dependent upgrade paths, so if you build lots of industry producing high-tech components you'll get laser turrets, while if you have lots of ammo factories you'll get ballistic turrets, rather than just spawning set turret types.
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Just thinking out loud here, but:
Would it be feasible to support more than 3 tiers without having to rewrite things? For some plots the step between tiers seems to be larger than others, and it could be helpful to have more stages to spread things out more.
Would it be possible to support a per-plot 'brown-out' (as opposed to turning off a random set of things entirely (usually water purifiers, as one of the single most expensive power using things) until you build enough generators)? ie. if total power demands are too great, start scaling back individual plot power consumption (and production) until requirements are met or everything is running on minimum power. Ideally this would feed into the plot animations/sandboxing. A plot that's only running at 50% capacity might only puff half the amount of smoke it usually does, or the settler might shut things down and wander off half the time (similar to a vanilla farmer farming less than 6 units of food). Obviously a plot that doesn't have its power needs met isn't likely to upgrade.
Actually, taking the previous a step further, could there be orthogonal upgrades, depending on what else is in the settlement? eg. To get turrets on your martial plots you'll want an ammunition factory in order to produce the ammunition the turrets need; the ammunition the factory dumps into the workshop is logically the excess after feeding the turrets and settlers guns; perhaps the ammo it makes relates to the turret and settler guns. Ideally each of the various forms of industrialisation would have some sort of impact on each of the other plot types.
I initially expected (especially given some of the phrasing) that we would use plots for everything, even if only purely from the point of view of laziness. I'd kind of expected a 'utilities' advanced industrial plot that's got a small generator and a hand-pump well, which slowly builds more generators and or/wells to support population growth, eventually it would unlock a power house/water treatment plant, and then eventually a miniature nuclear power plant.
I'd actually expect resource generation on the whole to be much less than it appears to have been so far, as you no longer are consuming much of those resources to build the settlement, the only sink for those resources are in equipping the player, or explicitly equipping your companions or the settlers. Perhaps alternatively, either logical resources generated by the industrial (or perhaps all) plots, which are then consumed to actually perform all upgrades, or actually consume settlement resources to perform upgrades (with a limit of having at least, say, 200 of any particular resource remaining after upgrades as a buffer).
Given that settlements are currently self-contained as far as Sim Settlements is concerned, it clearly couldn't work that way, but I'd expect that a steel mill would require having n (6-ish?) mines in the settlement network to produce enough ore to feed the mill. For a self-contained settlement that would prove too expensive.
I know that the vanilla system doesn't do it that way (and I don't know how flexible your assignment system is), but would it be feasible to have settlers work part-time on multiple plots (probably only tier 1, as higher tiers would presumably need specialised knowledge gained from looking after the tier 1 plot)? eg. This would allow you to zone power, water, food, defence, and have a lone settler handle all four (without going I'm thirsty/hungry/scared/etc.), as the amount of effort for producing enough power, water, food and defence for one settler is well within what one settler can do; perhaps set power/water/food to cost 1/6 settler-day per unit of resource generated (logically higher tier plots could be more efficient due to automation (eg. electric pump/purifier), or just being more reliable). Eventually you'd have enough work that there would be settlers dedicated to each role, but you'd likely still have casual workers taking up the slack (as opposed to working 1/12 of a day then wandering around going "Now what?" the rest of the time).
A related thought is that a martial plot ideally needs 3 settlers assigned to it for its full defence value (at least before automated defences) to provide 24-hour coverage, the ideal long-term watch schedule is something like 4 hours on, 4 hours off, 4 hours on, 12 hours off, but you could only achieve that with multiple guard posts and multiple assigned settlers. If you go with part-time work, then until automated defences go in any slack could be consumed by manning the guard posts (with the associated impact on defence values).
At least to start with I've found the sensors rather expensive, but later on with a large industrial base they're proportionally much cheaper, not quite an afterthought given the high-tech components required for their manufacture, but I didn't find myself running out of sensors to build up a new settlement, only to discover I lacked the resources to craft more. I think that I would find it more fun, though, to be able to just zone stuff, but that a settler building a tier-1 plot either consumes a sensor, or crafts one from settlement resources; or perhaps a cost in caps to zone more stuff, given you collect taxes?
One thing that I'd expect to be able to do without costing another sensor is re-zoning a plot, or is the sensor being consumed logically part of the cost of re-zoning a plot? If it wasn't, I'd expect to be able to re-zone a plot by interacting with the sensor; I imagine that there's an engine limitation, eg. moving from an plot not connected to power to one that requires power would likely be non-trivial to do, as opposed to just crafting another one.
One more thought that occurs is that before an upgrade is completed, it should check that it won't cause problems after the upgrade completes, eg. not having enough defence post-upgrade.
Perhaps then instead of the meter being 'plots with power', or 'do you have enough defence now', it should instead be 'number of plots that could upgrade without needing more food/water/power/defence/...'? A tier 3 plot would either not count, or count as a plot that could be upgraded, if only so a settlement that's fully upgraded has all of the meters full. It would offend people who insist that all the meters be full at all times, but it would make it more clear what a meter not being full means and if you should try and do something about it Right Now or not. Perhaps a two-part meter, eg. left half is 'current needs met' and right half is 'future needs met'?
It would mean that we'd want more meters, eg. once the first martial plot goes in, it adds a new meter saying you need more industrial plots for it to upgrade.