Here’s my own attempt at simplified grappling rules.
Treat grappling as a normal attack roll opposed by the opponent’s attack roll. (Armour doesn’t help you avoid a grab, but being a skilled combatant does.) If successful, you deal unarmed strike damage, and are now grappling with the opponent, and thus move into his square. By succeeding at an opposed attack roll, anyone in a grapple may perform their action: deal unarmed damage as an attack, draw a weapon, move half speed along with opponent (as a move action) or break from the grapple (as an attack), amongst other actions.
To further simplify, we ignore pinning. Initiating a grapple provokes an Attack of Opportunity, unless you are considered armed when unarmed (Improved Unarmed Strike). The feat Improved Grapple gives you a +4 bonus on all opposed attack rolls made in a grapple. Size bonuses still apply, as do the usual restrictions of grappling: no big weapons, no moving, and no Dex to AC from other non-grappling opponents. In addition, since you’re touching your opponent you can ignore miss chances such as blur and displacement.
Disarm and sunder can be treated much the same way - straightforwardly, opposed attack rolls. No attacks of opportunity are made, but you suffer a -4 penalty if you do not have the required feat. Like grappling, however, attempting this manoever when unarmed still provokes an attack attack of opportunity like any attack does.
How do these rules change the game? Aside from simplifying things for players, this grappling rule gives advantage against opponents who have particularly high Armor Class, have weak melee attack, or rely on large weapons, Dexterity to AC, or high mobility. The disarm and sunder rules meanwhile should see much more common use, especially against numbers of weaker humanoid opponents, when disarming or destroying their weapons to cause a rout may prove a convenient and heroic way to save a few hit points.