On cost of supercapitals: Once upon a time CCP projected that supercapitals were so expensive that this alone would limit their number to a handful. Doing that again would solve nothing.
Similarly, making it more risky to use titans would mean that they simply would not be used except when odds were already heavily in the favour of the side using titans. A suicide module for a 30B ship? Not going to get used. Killing a battleship, after insurance, is a few tens of millions of loss. This means that nothing short of killing over a thousand battleships at once would be "cost effective" for the folks watching K/D ratio and other important things like that - and a few people using such modules "for the lulz" simply isn't enough.
So, in order to curb the amount of supercapitals ingame, there needs to be a reason to put them into danger (and risk having them killed), there needs to be a way to kill them and, more importantly, it must be possible to limit the amount of them being built. Or, offer something better so the supercapital pilots will move to that instead of using their supercapitals.
A lot of this could be done as a part of the POS revamp. One of the more important things is the possibility of removal of the starbase force field, therefore making it possible to potentially surprise a bridging titan. Of course, allowing the ship to be anchored / berthed in a future starbase (for example, password protected to allow only the owner to board it) would also encourage people to use their supercapital character to fly something else. Main thing would be to offer something fun to fly instead of the supercap. Of course, it might be possible to still attack the starbase. This could be combined with the always-in-space supercapitals, meaning that in order to procect the ship while not online, it would need to be berthed. If it were possible to insure the berthed supercapital, it would again lessen the risk to use one in combat.
Other parts could be straight nerfs. You've no doubt heard the usuals, such as changing EW immunity to simply very good sensor properties.
And finally, limiting the possibilities to build a supercapital. Require a rare material. For example something that comes out of a once-per-character COSMOS arc, or something with an otherwise limited drop rate (has a chance to drop only from officers, nullsec Sansha incursions or a nullsec epic arc). One interesting thing would be if the materials to build one of the required components would be possible to get only by salvaging something specific - such as very rare nullsec and/or W-space sites or other capital ships (say, a "ship AI" which would be constructed using "shards of already experienced AIs"). It would be ideal if the drop rate could be controlled by CCP. If it's a random chance to drop from capital ship destruction, then it would simply increase the price of the supercapitals - which we have previously agreed to be only hindrance, not a barrier to the growth of supercapital ship numbers.
So, my path to limit supercapital ship numbers would be:
1) Introduce a rare component, limiting the availability of new supercapital ships.
2a) Revamp POS. No shields, allow berthing supercapital ships.
2b) Berthing may require buffs so people trust their shiny toys to be stored there.
3) After people are used to berthing ships, make supercapitals permanently stay in space, therefore making berthing a requirement.
4) Add a "mothership" class, transferring supercapital ship properties (bridging, clone bay) to this class. Keep the requirement of rare materials (and therefore, the bottleneck) intact for the supercaps, but otherwise decrease the required build components to make them "cheaper". Potentially couple this with a slight nerf.
Hopeful end result:
- A new class of support supercapitals, the Motherships. Awesome and stuff, supercap pilots do have something to look forward. Build numbers limited by a rare component, therefore the amount of ships in existance can be controlled. Bonus if Mothership build components could be confirmed to drop from titans and supercarriers (at a rate where a few dead titans could build the Mothership rare component). They retain current supercapital features, such as immunity to EW. Motherships are designed from ground up to be "mobile stations", requiring some technology that doesn't yet exist in game, so getting here might take some time.
- Titans are still a class of supercapitals. Their role is fighting, and they retain the doomsday and guns. Introduce doomsday scripts to alternate between capital-killer beam and a "remote smartbomb" (perhaps slightly more powerful than a stealth bomber bomb, but targetable) and possibly other scripts. Titans have great stats, but are otherwise suspect to EW (even a very lucky ECM attempt) and are otherwise the same, but with reduced endurance etc.
- Supercarriers are now big carriers. They can also dock. Their speciality is fighter bombers.
- Regular carriers and dreadnoughts are alright.
The goal would be to transform some of the current supercaps to Motherships as well as curb the building of new supercapitals. After having the New Awesome Thing, it would create less resistance to nerf the Current Pwnmobiles.