1) The AC ritual needs to be changed to be relatively better for more effective armor types. I suggest:
- Cloth armors: No benefit
- Anything whose non-mithril form is light, i.e. chain shirt: +1 AC
- Anything whose non-mithril form is medium, i.e. breastplate: +2
- Anything whose non-mithril form is heavy, i.e. full plate: +3
- Same for shields... Light+1, Heavy +2, Tower +3
2) Add an enhancement line that raises AC while using a Tower Shield. Call it "Tower Shield Defense" or something.
3) Add a Sword and Board feat that affects number of attacks. There should be an "Improved Sword and Board" that gives one extra attack, and a "Greater Sword and Board" that gives another, for a total of 2.
NOTE: This would still leave the maxed out S&B build with 7 attacks vs. 10 for TWF. So, there would still be a lot of reason to go TWF (30% more damage capability). But this would allow S&B to at least be viable.
4) Add a feat to make the shield bash act like a second weapon. I.e. you could attack with the weapon as main hand, and shield as second hand. The shield would not be as good as almost any available weapon, so again this would not be a replacement for TWF. But it would help keep S&B viable. (And the existing TWF feats could apply as an extension of the "shield as off-hand weapon feat.) Basically, for the cost of an extra feat, you could have the equivalent of a crappy off hand weapon while maintaining the S&B AC.
Many of the above should probably be Fighter class feats, to help balance the value of the Fighter class.
5) Add a S&B defense feat that gives you a chance to avoid getting hit even on a 20. Perhaps, when hit with a natural 20, the opponent has to roll to confirm the hit at +10 or something. (The S&B equivalent of evasion)
6) Add a feat that gives heavy fort when wearing Full plate and a tower shield.