It's pretty funny that you have the know-how to create addons for WoW and yet aren't that big on them. : ) DruidStats does most of what you're talking about, maybe all of it depending on your exact meaning, and does it well. It's at the top of my short list of favorite addons along with Outfitter, SmartBuff, RangeRecolor and Auctioneer.
It adds to an item's tooltip exact numbers for how much AP, armor, HP, crit, mp5, spellcrit, etc. an item will give you, taking into account your current form and talents.
You say "compare them to determine which would increase your base dps more."
"Base DPS" is just a function of attack power though, right? Not taking into account crits? If that's all your talking about then load DruidStats, look at the AP numbers and call it a day.
It might be interesting if you could come up with something that rationalized AP and crit into a "Ballpark Total DPS" figure for cats, and rationalized armor and dodge into a "Mitigation Value" for bear form. I don't even know if it'd even be logically possible to do so, but that's not my problem, I'm in marketing. : )
And even if you do all this you'll still hear "is X better than Y" because between AP, crit/dodge, HP and stamina there's just many factors to consider in gear...in most comparisons one item will give you more of something but take away something else. Is adding 15 AP worth losing .25% crit? Is adding 500 armor worth losing 400 HP? Not only does it depend on what you want to do, it depends on your current numbers. Maybe you're low on AP but doing fine for crit. Maybe you're armored like a tank but don't have much stam.
As such, I propose that we all come up some other Druid add-on for you to spend your time on. : )
/think
/thinkharder
Damn.
Oh well, I'm sure somebody will have a recommendation.