Well, I'll stop before I enter badwrongfun territory but whenever I see someone arguing about realism and D&D in the same paragraph I cannot help but chuckle.I don't consider it realistic that every party of adventurers has a rogue, a cleric, a wizard, and a fighter.[...]
The main reason I came up with this idea is that it is more realistic, and thus hoping to guide the game into being focused on role playing more than mere statistics.
D&D is about everything _but_ realism. If you are really interested in realism, why don't you pick a different rpg system? Harnmaster, Ars Magica, Runequest, Rolemaster, etc. - any of these would probably be a better fit for your preferences!
The common counter is of course 'but D&D is flexible enough to support any play style'. To this I can only say, yep, a swiss knife can be useful in many situations but it cannot ever replace a proper toolbox.
If you still want to go forward with your plans (and I'm pretty sure you will!), I'd advise the following:
1. Make sure your players are on-board with your ideas. Unless they're as interested in your idea of realism as you are, there's going to be trouble.
2. Be prepared to adjust your encounters to allow unorthodox party compositions to survive.
As a parting comment:
Have you considered that it is actually the result of 'natural selection' that most (successful) parties consist of the four iconic classes?
It's simply what works best, that's why _I_ consider it highly realistic that most parties have a similar composition.
In my games I allow my players to play whatever they want. If that results in a dysfunctional party, I point out the potential problem but ultimately don't care, because it is a self-correcting problem:
Party members will die until they end up with a team that works well together and is able to deal with the variety of situations that may come up in a typical adventurer's career.
Of course, experienced players will realize this right away and take care to work together when creating new characters to make sure they will end up with a viable party.