How do I automatically enable Personal Hotspot when my Bluetooth device is in range? (Similar to Tasker for Android)
There's a way to do this if you're jailbroken with tweak called Activator by Ryan Petrich, and unfortunately it's the only solution because regular App Store apps aren't allowed to access settings like Bluetooth, Wi-Fi, and Personal Hotspot (even Workflow cannot access them).

If you happen to be jailbroken, the way to do this with Activator is to go Settings &gt; Bluetooth &gt; On then Settings &gt; Activator &gt; Anywhere &gt; Bluetooth Devices &gt; Connected to [your Bluetooth device] and assign it to Switches &gt; Hotspot. You might also want to assign the Disconnected from [your Bluetooth device] event to the same action, so when you connect Bluetooth, your hotspot turns on, and when you disconnect Bluetooth, your hotspot turns off. If Switches &gt; Hotspot doesn't appear in the list of available assignments, then you'll also need to install Flipswitch by the same author.

For use on iOS 9 and 10, see Doron Gold's comment below on how to set it up.