With iOS 8 and OS X Yosemite, Apple made Spotlight Search smarter and more relevant by suggesting nearby places to eat, websites, apps from the App Store just to name a few. It's clear that Apple wants to make Spotlight Search the single place to start your searching on iOS and Mac. However, I think Spotlight Search still has much more potential, and it starts by breaking the restrictions of being a mere search engine. Google search, for example, is a search engine but, it can still show you local weather and solve simple mathematical calculations using dedicated widgets. I'm not suggesting Apple should develop it's own search engine but, continue to improve Spotlight Search with the goal of accomplishing more with less effort.
One of the features that usually distinguish third-party calendar apps from Apple's Calendar app is the ability to interpret natural language and create an event out of a simple sentence. Surprisingly, Apple already does this, on Mac only.
In fact, typing "apple special event on october 16 2014 from 1pm to 3pm" into the OS X Calendar app's Quick Event feature will produce exactly what you'd expect:
Of course, two obvious limitations to having Quick Event in Spotlight Search on iOS are Spotlight Search can't be accessed on the lock-screen nor within an app. However, in a situation where you just want to create an event quickly and avoid the menus of the Calendar app, being able to type into Spotlight Search can prove to be quite useful. Siri comes to mind as a solution to the problem however, you may feel uncomfortable speaking out-loud to your virtual assistant at some locations. It also happens that, creating Calendar events isn't the only feature Spotlight Search can borrow from Siri and — in some cases handle better.
The first is an obvious one, defining words. It's a strange omission. iOS already has a built-in dictionary so it can't be very difficult to start accepting "define" followed by a word in Spotlight Search. The same can be done for thesaurus and translating words. Typing a city or country name in Spotlight Search can bring up weather information. Also, by referencing Wolfram|Alpha, Spotlight Search can tap into a vast database allowing Spotlight Search to answer questions without a Google search (which I'm sure Apple would love) or perform complex mathematical calculations. For example, Spotlight Search can begin solving simple mathematical equations like 2 + 2 or the square root of 16 using the built-in Calculator app and then use Wolfram|Alpha for more complicated computations like solving for a variable or integrating a function. Again, Siri is already capable of doing that, but it makes less sense to use your voice when exponents and fractions are involved. Side note: typing "2 + 2 =" into Spotlight Search on the latest version of iOS will display a Wikipedia entry titled "2 + 2 = 5" as the top result. I though that was funny.
These are just a few starting points to improve Spotlight Search. It would be great if Spotlight Search opened up to third party developers too, but I wouldn't expect this to happen any time soon, considering Siri still can't interact with any apps or services unless predefined by Apple.