Some considerations:
With the increases in processing power since SFC I came out, I could see an SFC IV with the ability to handle a full 11-ship fleet engagement. If you're counting on "proper" human play, that's maybe a 4th player per side. With the "proper" multi-player code inserted, that should be no problem, as many FPS games can run with 64 humans at once... on a lot more detailed maps with characters having, IIRC, as many polies as a typical medium-polied SFC ship...
By coding in all the SFB S8 rules and other fleet rules, that takes care of the problems with the various weapons. Per SFB, the ISC is limited to 1 PPD bearing ship per every 3 ships in the fleet, and are limited to something like 8 PPDs total, which is coincidentally the most you can get anyway (4 from the DN / BB, 2 from the CC, 2 from the CS, with a HSC, CL, 3xDD-hull (one being DDL) and 3xFF hull (maybe swapping an FFL for the DDL, as you can't have a 4th command boat in this formation), rounding out the Echelon.
The only other HW with a "weapons restriction" on it is the Webcaster. Otherwise, all other HWs are unrestricted, primarily because they rely on luck and/or have counters (ie, an all-hellbore fleet doesn't have the fighter coverage of a mixed fleet or the in-close threat.) Also, these weapons are fairly balanced against each other (ie, firing every hellbore on a fleet does about as much damage as every photon in a fleet).
To me, SFC IV should be as close to SFB as possible, not because I'm a bible-pumping SFB-ite, but because a number of problems that have existed since SFC I are due to differences in the rulesets. Ships fly faster because they don't have to pay for EDR repairs or shield regneration, which messes up seeking weapons. Auto-shield regneration messes up saber-dancing. Nobody says the rules have to be exactly SFB (ie, the "EDR" magic screws can be figured by taking 2/3rd the number of systems that EDR could theoretically repair and making it 4 power per turn to repair the system instead of 3, or shield regerneration is factored by how much power is set aside to regeneration, ie 2 power = 1 point on each shield facing every third turn, while 12 power = 1 point on every shield facing each turn).
Hellbore fighters also had restrictions, as did various fed fighters, which the game should also restrict somehow.
One thing I think was a mistake was evening out the power curve for plasmas. the 2/2/5 arming cost for the R plasma (and other plasma arming costs) did interesting things to ship speeds, i.e. you could rearm at higher speeds, helping you avoid counterstrikes. But when it came time to finish arming, your speed would drop a bit, and you had the decision 'do I fire now or wait until the holding costs kick in?'. Now your speed is constant so that little tactical nuance is gone.
As for EDR, etc. I have an observation. When you were gathered around the table for hours on end, and a half hour or more passed between volleys, how damage was handled was OK.
In SFC, where engagements can be over in 2-3 minutes under the CURRENT rules, I'm sure the designers realized that leaving damage at SFB levels resulted in ships dying waaay too quickly! If your ship always disappears with the first volley, well the game won't be as much fun frankly. I for one think that doubled internals is a good thing, as it allows the game to play out longer, and keeps weapons on board longer, so players are able to fight back more effectively after a crushing volley.
As for SFC damage repairs, everybody gets similar benefits from it. You can argue that some weapons are hurt more than others, but the way it works now does make the game more appealing to newbies. As it stands now, you can stay in the fray while doing repairs. By assigning an energy cost to repairs, this encourages people to break off to do repairs (which I used to do in SFB), resulting in stern chases, etc, hence MUCH longer games. And we've heard enough people complain about how long plasma ballets can take...
As for shield regeneration rates, I again think they are a good thing, as long as they aren't too uber for any given race. The reason disruptors have difficulty versus the feds isn't so much the fact that the shields are regenerating, but rather the fact that they are regenerating faster than 2 points per turn on cruisers (3-4 per turn for DN's and BBs), as they would in SFB with damage control rates. I think having labs dictate how fast shields regenerate has been a recipe for game inbalance. I'd rather see system repair rates tied to the number of labs onboard, to sorta kinda reflect that EDR thing. Feds had more labs, so they could make repairs more certain than klinks with their measly 4-6 labs. this also reflects design philosophies: Klink designs focus on taking you out quickly (better firing arcs), while feds are designed to soak up damage a little better. So, I think that shield regeneration rates should strictly be a functiion of your damage control rating.
And then there was CDR (continuous damage repair) as well, which took a while but didn't cost any power. 4 repair points a turn could get you a phaser 2 back in one turn as I remember... which the Taldren system sorta kinda reflects (minus the part where you could only repair up to your damage control rating). Perhaps to meet you halfway you could have a 'repair' energy category to speed up repairs (to reflect charging up your labs), and the amount of power you could assign would be a function of how many labs you have.
As for spare parts, I think that what people carry around in any given battle is probably too high. I think I'd rather see this tied to your damage control rating (and perhaps labs), with a small 'spare part' regeneration rate between engagements, instead of packing around 20-30 parts in a given engagement. This would reflect supply freighters, pirates, etc. that you rendesvous with between battles. Perhaps 2x your damage control rating plus 1 per lab onboard? And possibly replenish them at a rate of 1/2 your damage control rating when in home space? This would make raiding the parts stocks on enemy ships a little more important when in enemy territory...
Batteries need to be implemented much better than they are. At the very least, I think a check box or something which automatically drains batteries (for specific shield reinforcement) if you are about to take internals would be something... What would be an idea would be to have some way to assign batteries to specific energy uses (i.e. I want to dump batteries into weapons only, but not ECM, shield reinforcement, or whatever). Again, this would be a check box on the energy distribution panel, so when you flip batteries on, they would be split between the categories that are checked.
It would also be nice to have a 'sixth' energy category for regcharging batteries. That way, you could, say, have movement at priority #1, battery recharge at #2, weapons at #3, and so on. This would really make batteries a lot more useful, as they were in SFB.
It would also be nice to be able to repair batteries. And perhaps hull, lab, and other systems as well...
I don't think that game imbalance in SFC is really all that bad, as the player distribution fits the pattern. We have a lot of feds, because they are the feds after all. We also have a lot of klinks, and the other races, while having less players, are more or less distributed somewhat equally.
I'd argue that droners have more fans though (Mirakzinti), what with the KOTH, KAT, and other fleets. Suprisingly, we don't seem to have a lot of ISC players though (I could be wrong), which is odd since everyone seems to consider their ships more uber than everyone elses...
If anything, the faster missiles in SFC are much scarier than the SFB drones... when was the last time you took a speed 8 drone seriously (outside of an anchor situation)?
Cloak needs some tweaking though... this is obvious based on the low numbers of people that still play Rom. As for plasmas, well many of us said a long time ago 'give us direct fire plasmas and they will be fixed'.