OK, so if we treat Cleric, Wizard, Sorc, Fighter, Rogue, Druid, and Barbarian as done, we're left with Paladin, Bard, Monk, and Ranger. Any suggestions for capstones for these? I'm not massively concerned since there are lots of feats out there designed for these classes.
So we're sayin that arcane thesis is the capstone for wizard, DR 1/- for barbarians, Martial Veteran for fighters, a rogue special ability for rogues, 3rd level spells for sorcerers, and access to the domain feats for a cleric. I think I missed the druid capstone. Was it wildshaping into large creatures? How about for a paladin something called aligned strike that treats all of their attacks as good aligned? Give inspire courage +2 to the bard. Give the monk wholeness of body or improved evasion. For the ranger give them an ability called favored strike that gives them a +1 to attack rolls versus favored enemies and allows them to overcome DR of favored enemies. It may seem like I'm giving the ranger a lot, but unless they are lucky enough to pick favored enemies that will come up a lot in the campaign, they only get to use it about 20% of the time.
The really simple way to have capstones for the ranger and paladin would be to give them an extra favored enemy and smite per day. That might be better than coming up with new class abilities, since we want to keep everything in line with PHB as much as possible.
I would be tempted to give the monk and ranger a class feature they wouldn't get otherwise due to the 6th level cap.
Perhaps Wholeness of Body and Woodland Stride respectively. Those would be what they got at 7th level anyway, and probably wouldn't be too out of balance (although Swift Tracker might work for a ranger instead).
For that matter, you could go ahead and give the bard his +2 to inspire courage that he would have received at 8th if you didn't think it was too overpowered (or even inspire greatness).
You might consider giving a paladin a "Holy Strike" similar to a monk's Ki Strike. All melee attack are considered good aligned for the purposes of overcoming damage reduction against evil outsiders and the like.
Eventually, sure. But only when the best feats have all been taken, which won't be for a while. If a fighter capstone feat is the ability to treat his BAB and fighter level as two higher, then there are quite a number of excellent feats and feat chains that he can complete. And the +2 to Str, Dex and Con will take 6 feats in themselves. By the time everyone has 30 or 40 extra feats then, yeah, sure, those extra 4 feats won't be very special. But when there are only 12 or so extra feats? Those 4 extra are still good then.
I'm curious as to how the E6 rules work with the Gestalt rules? Could one use the Gestalt rules in conjunction with the E6 rules to create even more powerful characters, but still maintaining that "realism" limitation?