This is an example of how to build AD Sion that has been successful for me so far in Dominion matches. Its built to maximize his damage during stun duration enabling him to dive defenders as well as guard after capping instead of recalling. It is simple to play, and I recommend him for any beginner players getting their feet wet in Dominion.

The runes I've chosen are to maximize the strength of the 15% armor pen and the mana regen you already get from the Dominion buff. The glyphs could be changed to MR or MR/level if you dont have CD runes, but the magic pen buff everyone gets makes me feel CD is better for Dominion. Movespeed quints are to give you that slight edge on a map made for speed.

Your favorite version of 21/0/9. You want to kill. Fast. Global vision at every point means you dont have time to harrass that solo defender. Assume backup is on the way at all times.

Garrison is amazing. I have it up at least 4 times a match, unless one side 5 caps and ends it early. Its essential for tower diving as well as solo defending. For anyone who hasn't bothered to use it yet, it works like this:

When diving 1-2 champs, stun one, pop shield and ult(if up), garrison the tower, procede to kill the squishier of the two champs. They will either puss out and leave their mate or die. Kill or rout the second one. You have plenty of time, AD from your E, and attack speed. The tower only does 20% of its damage with a chance for your passive to block 40% of that for you.

When defending, stand and fight if its 1, feign retreat if its 2 or more. They will try to cap. Pop Garrison and it will interrupt their cap, plus immediately start blasting them in the face. Stun the one that is targeted by the tower, and he is a goner. The other one will run, they have no choice with Garrison up. Your too beefy and you have lifesteal.

I am basically going for speed with this build, both in attack speed and movement. I am also relieing on lifesteal for defense. Its been working so far, even with the debuff to healing on this map. I don't feel you need AD items early on since you have it coming from your E, but atk speed is essential IMO. You want to maximize the amount of damage/defensive lifesteal your getting from your stun/ult combo.
I tried using the magicdmg proc item first but wasnt impressed with the damage vs just going more AD/Lifesteal. Your stun time is basically your time to build back health in-fight, and your shield you use when stun runs out. Combined with lifesteal items and passive, I've had my way with every champ 1v1, and 1v2 usually one will run and then turn and fight. Exploiting that fight or flight response is the key to every 1v2+.

Its not Summoner's Rift, but its also NOT Arathi Basin. Power levels due to gear change in Dominion, but there are no twinks, no epics, and no mounts.

You also don't need to give up at any point because you can ALWAYS win even with 10 points to their 100. You need 3 cap points to win. Thats the name of the game, thats the only thing you need to remember. This may change down the road because its actually pretty ridiculous, but it the current state of play. If you have 3 points capped, your nexus will lose NO points (except due to death), and your opponents will (lose nexus points).

Thats all for now, thanks for reading if you made it this far, and good luck!