Lately I've been thinking about what would be the most fun build for Tristana.
I've been thinking start with doran's, of course, then berserker's greaves, then infinity edge, phantom dancer, ionic spark, force of nature?
Not entirely sure about this, if anyone has any ideas, in particular Mr. Wallet, I would love to hear them.
I'd say skip the greaves. Her Q is a attack speed roid, you should focus more on damage.
Also ionic spark is a bad idea. You don't need to be dealing small multi-target magic damage when you could be putting that towards ripping apart people one at a time.
Force of nature doesn't look like a tristana item at all. You should at least have a blood thirster in there. I sure as hell hope you're not going to be a bot and keep that Doran's blade forever. You've only listed 5 items after it.
So yeah, ditch the ionic spark and force of nature, let yourself know you can sell the doran's blade for a 3rd replacement later game.
So consider these items:
The Bloodthrister: 60-100 AD, + 15-25% life steal. It's every AD's dream how can you not want it? Even if you can't manage to farm 40 minions before dying (how can you not do this with explosive shot's passive?), it's flat stats are still great.
The Black Cleaver: +55 AD, +30% attack speed, +0-45 armor entry. Attack damage, attack speed, and armor shredding. Good for if they're building a little tanky.
Banshee's Veil: +375 Health, +375 Mana, +50 Magic Resist, 45 second cooldown shield. The 50 magic resist is a blessing for fighting your worst enemy, the AP carry. These fellas usually like to jump in, dump everything on you, and jump out. With the 50 MR, 375 health, and blocking their first spell, they might actually think twice about doing this. You can strike fear into an AP carry just by having the blue bubble around you. They rely on having their spells ready. They sure as hell don't want to waste one to do nothing. Note for damage over time spells, it will only block the first tick. For things like Malzahar's ultimate, the channel will stay on you, but you will not be suppressed.
Another The Bloodthirster: more AD and lifesteal, enough said.
Madred's Bloodrazor: +30 AD, +40% attack speed, +25 armor, 4% enemy Max HP in magic damage. Good for if they're stacking health items. Even better if they're stacking health and armor and not magic resist. The armor will also help for fighting their AD carry. It's even got some AD and attack speed for your autoattacking needs.