Special type of T2 Fusion, Underground, able to withstand a nuke at least 2 hits, and or bombings. The beauty is you simply slightly change the current units look a little bit, and change their elevation and add them, have 4X the hitpoints of an aboveground, but cost 3X to build.
Setting aside the underground aspect, I do think the idea of a big, hardened structure that produces energy is actually a good idea. Various mods of TA have actually tweaked the fusion to make it function quite differently from how they were designed in TA. Including making them basic structures that produce energy more efficiently for cost, but are obviously a liability because a single fusion is much easier to destroy than a solar field, and fusions also blow up. A fusion could be a straightforward dedicated and efficient energy production structure. Expensive and high yield, so you have to delay when you start building them since you need smaller, cheaper energy early in the game. Fusions could even be made available to basic constructors, relying on their cost to determine when they can be constructed instead of tech. Which brings me to your idea, which is a more specialized fusion with other benefits than only stable energy production. Advanced constructors, instead of having access to a strictly superior energy structure, could have access to a hardened fusion that is considerably less efficient for cost, but vastly tougher, to the point that it can survive nukes. TA had cloakable fusions, which are quite close to your idea for an underground fusion. However, in practice this just makes isolated fusions hard to find, and might be a bigger problem in PA. So instead, what about making a large, very tough structure that can survive a lot of punishment, which produces as much energy as a fusion. In fact it could even have other functionality as well, including limited defenses, and perhaps even the ability to load units (such as a Commander) to protect them. Such a structure would need to be expensive, so if you're only interested in energy production you should still build fusions.
That sounds too fancy, but I like your idea of a 'hardened fusion' basically it's the same fusion with heavy plating able to withstand at least 2 direct nuke hits, and various artillery / air bombardments, of course build cost should be double in compensation
As interesting as underground stuff might be, the biggest problem is making sure the game field is visible and easy to use at all times. No one really knows how to do underground stuff without switching screen modes a billion times. De-compiling Ledarsi's posts is fun. Basically 4 paragraphs got you a super tough fusion or a cloaking fusion. Cloaking fusion is messy. Energy is the mechanic we use to limit who can cloak and when (but currently not where). An energy structure that cloaks itself is suddenly not limited in how it cloaks on one side. Making energy to spend it directly at the source is a colossal waste of resources on the other side. Tough fusion is nothing special. A high value unit is given more HP. No, it doesn't take a completely new unit to realize this.
I like the idea, just not the underground part. We already have ground, air, naval, orbital, and multi-planet. We don't need yet another layer.
My point is that features like the OP's can be used to differentiate advanced energy generators without just making them strictly superior. Instead of having just a strictly more efficient energy production structure available to advanced constructors, advanced energy facilities might offer other benefits. For example, an advanced hardened fusion could be very tough. I agree that cloaking/underground hiding are probably not good features, but there are lots of other possibilities. The best basic energy production facility should probably be the most efficient one for pure energy generation, at any point in the game. But the advanced ones can give you other tactically significant advantages, other than just raw efficiency.
I really like your point about the basic energy producers should be the most efficient..... That is very important because it sets up the more advanced ones to serve for other purposes..... ex: stronger or more production at once.....
Why would anyone build a [geotherm] if the T1 generator is already the best? Resource structures are made to give resources. Try not to overthink this.
I personally think it might ADD to the game, an under-ground layer. Why wouldn't it work just like submarines do, but for ground and stuff? I would personally like to see it as some sort of.... "Eco Base" Layer, like how the land layer is the "Bulk" layer, the air layer is the "Harassment" Layer, the sea layer is the "POWAH" Layer, and the orbital layer is the "Gimmick" Layer. in my vision under-ground would have Geo-thermal mega reactors, which would produce lots of energy AND a little bit of metal on the side (Lava metal? is that a thing IRL?) while being quite a bit hard to dislodge, but it's pretty darn expensive, and the rest of the base pretty much has to BOW DOWN TO DA POWAAAH because it's pretty darn big. Also, maybe you can hide units in it? My two cents. YOURS TRULY, PLAGUE DOC
How about a kind of "armor plates" that you can add to existing buildings to give them extra HP? Ofc. the bigger, more expensive the building the more they would cost.
You build a base and go "Oh no! That guy is gonna bomb me!", so you add armor to your buildings. Walls don't help against bombers.
Let's see. Should I buy one defenseless building and throw a bunch of armor on it in the vain hope that my enemy will attack that and only that structure, or should I have TWO buildings, thus getting twice the health and twice the production/resources? ...
Who says it would double HP? Or do anything with HP for that matter? Who says it would cost as much as the building itself? Who says resource buildings would be the only ones you could use it on? How about a radar or anti-nuke?