1> Allow multiclassing, but only to the Racial Favored Class, and only as long as Monk is still the highest non-Prestige class. For Humans and Half-Elves, any one class can be multi'd with Monk, but if you ever add a level to a third non-Prestige class you can't go back to Monk.
Same goes for Paladins.
(If you do the "school" thing with different combos it's abuseable because you'll always come up with some new school that has exactly the combo you want)
2> The problem with a pure-class Monk is that the abilities are always the same, there is NO variation in class abilities. Rogues pick high-level abilities. Casters pick spells. Fighters pick Feats. Only Monks and Barbarians have no variation, and at least Barbarians can use different items.
So, you could do the stuff in Oriental Adventures, or add more Prestige Classes for them. In my world Monks weren't really religious (so the Domain idea, as good as it is, wouldn't fit), but were more Elemental. So, we tweaked the base class a bit, dropping a couple minor abilities for a new ability, Elemental Strike, that lets them change the damage type of half of their hands' damage to a single common element (Fire/Cold/Electric) at level 6 or so. At higher levels, they can pick more elements to choose from, stronger elements (Acid/Sonic/Psychic/Lawful), and/or replace ALL the hand's damage instead of half. Since they're picking abilities there's more flexibility. Keeps things interesting, and you have to admit, flaming fists is cool.
But, this is probably more appropriate for a Prestige Class.