I don't feel strongly one way or the other about seeing a stealth system added to the game. I will however say that any such system ought to be kept simple.
2) Stealth modules weight about as much as a engine module and are difficult to stack without sacrificing other stuff. Each tech level of stealth reduces detectoin range. tech level 1 means you can be detected from 5 hexes away, tech level 5 means you can be detected from 1 hex away (adjacent). Stacking lower level modules adds the bonus and stacking a max level stealth module is useless.
I would much rather have -X detection range stealth than 'detected within X range' stealth, because -X detection range stealth has an obvious counter (add more sensors) and just makes more sense. Detected within X range stealth doesn't make a whole lot of sense when you can have a ship with sufficiently powerful sensors to detect things 20 tiles away and a ship with just the bare minimum sensors detecting things 3 or 4 tiles away, and yet somehow your stealth system gets you within X tiles of either one, rather than within (sensor range - X) tiles or (sensor range)*(100 - X)/100 tiles or some other such thing.
Moreover, there's not an obvious way to add the bonus in the 'no detection until within X tiles' system - how do I add "is not detected unless within 5 tiles of an enemy ship" to "is not detected until within 4 tiles of an enemy ship?" Making stacking pointless is fine but the last sentence in the quote suggests that you had some kind of stacking mechanic in mind, and I just don't see an obvious, simple way of doing it. -X detection range at least has an obvious stacking mechanic if you just add the detection range reductions together.
edit: another problem with -detection range stealth to me seems that it would look really unnatural to have ships appear and disapear everywhere in your sensor range all the time, and ships not appearing at the edge of yoru sensor range, but well within it.
This couldn't possibly be more unnatural in appearance than the 'only detected within X tiles' system that you proposed, which has no relation to the capabilities of the sensor platforms in the region. At least -X detection range makes a degree of sense when comparing the effect on weak sensors to the effect on strong sensors.
This is actually a big problem for stealth in this game, because there are no ranged attacks you either cannot stay hidden while attacking (and presumably have less defense and offense because you spent space on your ship for stealth), or you always win. leading to one of the 2 scenarios above.
Civ II had subs which could not be detected unless you had a unit adjacent to the sub, and they worked just fine despite having to move next to their targets to attack.
Fleet-wide stealth would presumably require a fleet type module. If a particular ship in the fleet has stealth and hasn't been detected yet, then it wouldn't appear in the list of ships in the fleet. That could make for some interesting surprises and traps
I would very much rather that 'fleet-wide stealth' meant that all the ships in your fleet were designed as stealth ships rather than there being some kind of magic fleet cloaking device that you can mount on a flagship.
Just don't permit mixed fleets to occupy the same hex like it does now. If you enter/stopping-in a hex containing a stealth ship you could just get the Do-Combat message and show the ship/fleet--perhaps without all the details. Then you to decide what to do. Once you've started combat then you could see them until they left detection range. Same for if they moved into detection range and were actually detected, they would stay visible until they left.
I am very much against ships being undetectable when adjacent, or especially when sharing a tile. If you want stealth to matter in combat, go for something reasonable like a slight accuracy penalty to the attacker rather than something silly like space submarines.
Cloaking planets hmmm...how about making the entire map cloaked!
No thanks. Having the exploration phase occur once per game is sufficient, and it's simply ridiculous to assume that I'd lose track of where stars and planets are; if nothing else, I'll be able to locate them by their EM emissions for the next several years even if you do come up with a system that makes the planets and stars completely invisible to all types of sensors, and presumably my faction can at least give me a mathematical prediction of where the planets and stars are for a reasonable amount of time after you somehow made them invisible to every sensor in existence. Even not knowing about a colony is a bit of a stretch, especially since you actually want me to know about your colonies so that you can lay claim to systems, and perhaps attract tourists.
Beyond that, if you develop a system that makes it possible to take your empire off the galaxy map, have you considered what that would do to your economy? A truly effective cloaking system on that kind of scale would prevent even your own traders from visiting your worlds, preventing immigration to, emigration from, and migration within your empire (which would severely curtail colony growth), remove all your foreign trade, cost you all your tourist income, and likely wreck your internal economy if it had begun to rely on interplanetary trade for any reason.