It's really just JKR being unimaginative and by proxy everyone in the wizarding world. Because of what she's written, the three unforgivables are the crème de là crème when it comes to the dark arts. Which if you think about it only means they're so called evil. Why? The only thing that makes sense to me is that to cast them, you need to want to kill, control, cause pain. And because of this your soul or something gets tarnished until you become well, I have no idea what the soul is supposed to be like in JKR's universe. Say you wish to kill muggles, that thing pettigrew did, 13 with one curse? Yawn yawn. Use your imagination. Go to the basement of a skyscraper and transfigure the supporting pillars into marshmallow before apparating out. Blast the wheels off a speeding passenger train while it's making a turn. Drop poison into the local water plant. You'll have hundreds and thousands and tens of thousands dead, with one spell.

But to the point at hand, the wizards are just plain unimaginative. If you think about it, with the magic in the HP verse you can do pretty much anything. Constant vigilance or whatever but because there are so many ways of infiltrating a public government you can't do anything against concentrated efforts. One second of inattention, being caught unaware and boom! You're suddenly turned into a security leak yourself.
And once you have infiltrated the government there are billions of ways of causing mayhem. You're not limited to avada to kill. Say you send in 10 imperioed stooges into the ministry, have them conjure or transfigure or carry hydrogen. You won't notice anything until the building is filled and someone lights up a tiny spark. BOOM! No more ministry. Any security measure could easily be taken care of if you control the people in charge of it.

Subjugating the british ministry as Voldemort should be a piece of cake.

Just off the top of my hat we have:
- Polyjuice potion, felix felices (sp?), invisibility cloaks, cosmetic charms, self transfiguration, metamorphmagus, animagus, death eater sympathizers in the ministry, compulsion charms, notice me not charms, imperio

Any of which we can use to sneak off with key people of the ministry while they're at home, shopping or even at the ministry to...

Use torture on, veritaserum, legilimens and my glorious presence to force out anything that could be of use. Floo adresses, work schedules, projects

And once we have this, infiltrating further and repeating the information gathering phase over and over again until we know what needs to be know. Without the public being any wiser.

Then when all preparations are done, the ministry is taken over fast and efficient throwing the wizarding world into chaos. Without a central government there is only pockets of resistence and if it's anything like during the first war in canon the rest of the world won't interfere. And even if they did, so what? They don't know who to trust because of course I'd still keep imperioed sleeper cells infiltrated amongst the other side. Secretly spying.

If you want to take over something you'd better be prepared to strike decisively or you'd just be overwhelmed with superior numbers.

 

Clearly, the only way to win was to bribe dursley with a couple million pounds to hand Harry over as a child.

Then just put him in a sack, tie some bricks to it and throw him into the Thames.

 

I wouldn't do disfiguring dark rituals, I would just make a horcrux, so that I was immortal. Once immortal, I would try and discover what would happen if I was killed while having a horcrux, possibly through experimentation involving Imperio. Once I discovered the result, I would then research how to regain a body within a convenient time-span. Once I had that completed, I would commence with my original plan, and create the Death Eaters, I would probably put the Dark Mark somewhere very inconspicuous, like the roof of their mouths. I would then use extensive Veritaserum and Legilmancy to insure their loyalty, periodically. After being absolutely certain of their loyalty, I would instruct each of them in how to make a horcrux, and how to revive yourself when killed.
I then have a loyal army of immortal wizards/witches, who could stop me?

 

Loyalties change. They are but passing infatuations.

 

For an immortal warrior, what would be the point in serving another? Sure he could kick your ass, but if you hide your horcrux well enough then its no problem at all. After a while you'd have every Dark Wizard from here to New Zealand (Dark Sheep in them cases) with at least one horcrux and in varying states of disfigurement. Wonder if you could make a chart to work out how many horcruxes a guy makes just by his physical features...

Aekiel