This is my first Build on here so go easy on me.
Shyvanna is a very strong hearty toon who can easily control top in 3's. I personally find her to be best when played as a hybrid tank build. This allows her to be very aggresive from the beginning and push out your opponent and in turn out level the whole other team.

Pros:
She has excellent starting damage output
Solid sustain when designed correctly
Disceptively difficult to kill

Cons:
Easy to get overconfident with
Needs good farm or will suffer by mid game
Tends to let people escape a little too often

I typically go with AS quints and marks. Then I use CD reduc glyphs because her abilities are always in need. Seals I am using HP per lvl, but I am going to try the new % HP seals.... I'll keep you posted. These runes are designed to emphasize her strengths so I can build items into her weeknesses.

I go with a fairly typical tanky attack build. The big tank aspect for her I try to establish is high HP..... Leaning back to the possible use of the % HP runes. Her Itemization is heavily concentrated toward AR so I am sure to max out MR on here. Once again keying in on her weekness areas of Cooldown reduction and movement speed. Trying to correct her inability to close out near kills.

The itemization is designed around having her wade into the group of either mobs or champs and burning them all up, while simultaniously killing one off at a time. Frozen mallet is critical for this she needs to keep those slippery champs from getting away. Remember her flame breath will proc the mallet so it is a super item for her. After mallet and sunfire cape it gets very situational. If you are the only non-squishy build more tank stuff for the typical Atmogs type build. If not then go for more AS and damage with stuff like Stark's and Black Cleaver.You will get plenty of kills.... don't worry, think about team helpful items.

Burnout is your bread and butter.... It gives you mobility and constant nice AOE damage... It is the reason for this build. Flame breath is second because it provides a nice armor debuff... not to mention is your only source of ranged damage. Other builds would probably put twinbite higher then FB but not for this type of tanky toon.

I choose two aggresive attack summoners because she has decent escape ability innately. In addition more often then not having both ignite and exhaust will get you early kills that you would not have been able to pull off without them. Early kills / lane pushouts are critical to making this build work. It is designed to be very aggresive early and often. Doing this will allow for a snowball down the hill effect. You will heavily out farm and level everybody else.... In turn ganking at will.

Shyvanna is a really cool toon who can be very OP if you learn to capitilize on her strengths...... Not to mention she turns into a friggin Dragon,which is awesome all by itself!

Have fun... tell me what you think AFTER you try it out.