An offensive PLD defeats the entire purpose of a PLD in an exp. party. Subbing MNK and DRK are horrible ideas if you want to party. WAR is the only viable sub as it gives the PLD voke. Without provoke they cannot function properly as a tank. Also, two-handed sword is only viable in certain situations and usually sword/shield is much better in a party situation.

You can do whatever you want if you're soloing, but that's only when you're soloing, otherwise you're going to be a gimp and seriously hurt the party by not performing to your full potential.

Soloing an offensive PLD is feasible. PLD has around the same base STR as a MNK and with the right STR and attack increasing gear can perform as a decent damage dealer. WAR will offer more STR than a MNK sub, though, and DRK is just a stupid idea. The two don't work together. In most situations DRK is just one of the worst possible subs in the game.

WAR is far more like the knight that you're looking for. WAR has tanking capabilities but also is much more effective damage dealer. Give a WAR a great axe and some good STR and attack plus gear and you can cut through things easily. DRK would be the next closest thing to what you're looking for except for sacrificing the ability to tank effectively.

Simply put, PLD are built to tank. They have some of the highest def and VIT in the game and the white magic only serves to help yourself in combat and take some of the load off the WHM.