Many people pooh-pooh the D&D alignment system as too restrictive, but I think that partially results from a misreading.
It's not that a monk must be Lawful, it's that a non-Lawful character wouldn't become a monk. It's not that a paladin must be Lawful Good, it's that only a Lawful Good character would become a paladin. It's not that a cleric must be within one step of their deity, it's that a person wouldn't devote their life to serving a deity with whom they substantially disagree. It only entails restrictions if you're looking at it from a game perspective rather than a character perspective.
That said, the "alignment is too restrictive" argument can be sound in rare cases. Especially if you read alignment as motivation-based rather than action-based, as Immanuel Kant and I do. A Lawful Neutral contractarian will wind up behaving in exactly the same way as a Chaotic Good utilitarian 9 times out of 10; the difference is in why they act the way they do: contractarianism is born of self-interest, while utilitarianism is supposedly born of legit altruism.
If I say "I'm a good person because I want everyone else to be good to me", that's selfish and clearly not Good. If I say "I'm a good person for the sake of being a good person", that's Good. The behaviour is the same, it's the mindset that's different.
On the other hand, in traditional D&D cosmology, the best reason to behave in accordance with a given alignment is that you want to go to your favourite afterlife. I wouldn't necessarily want to wind up in Mechanus, as it may be somewhat too rigidly structured for me. I'd probably be okay with most of the Good planes. So I would pick whichever afterlife I thought best (probably taking into account its neighbors; for example, Ysgard isn't terrible, but it borders on Limbo, which is, so I wouldn't aim for Ysgard, because if I miss Chaotic-Chaotic-Good and hit Chaotic, I wind up in Limbo, which is bad).
But that gets into the John Constantine problem: if you do good to get into a good afterlife, and alignment is motivation-based rather than action-based, you go to a bad afterlife because you're selfishly motivated, no matter how well-behaved you are. So that's an argument in favour of treating alignment as action-based rather than motivation-based.
All of that said, if you consider alignment too restrictive, or you treat it as motivation-based rather than behaviour-based, I made a feat for you. Along the lines of Monastic Training and Knight Training, which allow you to multiclass with Monk and Paladin without penalty, here is Versatile Alignment:
Versatile Alignment [General]
You’ve picked up enough habits outside your alignment that you are able and willing to behave in ways that conflict with your true alignment, and can take levels in classes you otherwise wouldn’t due to your alignment.
Prerequisite: +5 Base Attack Bonus
Benefit: Choose one class or prestige class. You may take levels in that class even if your alignment is one step different on one axis than the requirement.
For example: You may take levels in the barbarian, bard, or druid class with this feat if you have any alignment.
You may take levels in the monk class with this feat as long as you are not chaotic.
You may take levels in the cleric class if you are one additional step from your god. For example, a cleric of a lawful good god may now be chaotic good, neutral, or lawful evil, in addition to lawful, good, or lawful good. When doing a god’s work, you must still adhere in practice to that god’s alignment or risk losing your cleric powers.
You may take levels in the paladin class if you are good or lawful, but you must still adhere in practice to the paladin's code of conduct or lose your paladin powers.
If you take this feat after losing your class powers due to a change in your alignment, you must still undergo atonement before you can regain your powers.
Special: You may take this feat more than once. Each time you do, it applies to a different class.
An additional, even more optional option. For those campaigns which ignore experience penalties for multiclassing (or which, like mine, allow one extra free class before experience penalties start accruing), the Racial Favored Class winds up doing absolutely nothing. Every race has one, but it doesn't do anything. So consider this:
Every character gains the benefit of this feat applied to their racial favored class. For races whose favored class is "any", choose which class to apply this feat to at character creation. After that, it may never be changed.