Will there be a USB-C to thunderbolt 2 adaptor?
USB-C is a connector, Thunderbolt is a protocol, which makes this something of a nonsense question. USB-C can carry a number of protocols, including Thunderbolt and USB. Thunderbolt is a protocol used on USB-C and mini-DP connectors, if there are more external connectors than this in common use then I am not aware of it.
There are Thunderbolt mini-DP to Thunderbolt USB-C adapters available. Apple sells one. https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter
This is not an adapter to the USB protocol, any drive connected by this adapter will have to be a Thunderbolt drive to work. Having a USB-C port is not sufficient for this adapter to work, there are many drives that use USB-C but the USB protocol instead of the Thunderbolt protocol.
Thunderbolt drives with USB-C will likely be no faster or slower than Thunderbolt drives with the old mini-DP connector but they are likely to be cheaper. USB-C drives have a bigger market and this will mean lower prices out of economy of scale.
Using a Thunderbolt drive with a mini-DP connector with a computer that supports Thunderbolt on a USB-C port only requires that Apple adapter or something like it. Using a Thunderbolt drive with a USB-C port with a computer that supports Thunderbolt on a mini-DP port means using the same adapter, the adapter is bi-directional. Using a Thunderbolt drive with a captive USB-C cable, a cable that is permanently attached and a USB-C male connector on the end, will not work with that adapter. Or at least not with that adapter alone. Adapters with female USB-C ports do not meet the USB-C spec. They exist, can be purchased, but by not meeting the spec they can damage something. It's best to find the right drive without the captive cable.
I only need data transfer to be higher then USB 3.0
USB 3.0 will allow 5 Gbps, USB 3.1 will allow 10 Gbps. USB 3.0 and USB 3.1 can use the old USB-A and "wide" micro-USB-B connectors that are common on many computers and hard drives, or they can use USB-C. Any USB drive with a USB-C port doesn't make the USB protocol any faster, but it can make them more convenient because USB-C is a nicer connector than the old micro-USB-B.
Getting faster than USB 3.0 with a computer that has a mini-DP Thunderbolt 2 port, and a drive with a USB-C port, means using a Thunderbolt drive and a Thunderbolt mini-DP to USB-C adapter. If the drive does not support the Thunderbolt protocol then it will not work with the Thunderbolt port and adapter. It is possible for an adapter from Thunderbolt 2 on a mini-DP connector to USB 3.1 on a USB-C connector to exist but I haven't seen one. Using a Thunderbolt 2 to PCIe expansion chassis and a PCIe card with USB 3.1 on USB-C ports will also get faster than USB 3.0 with a USB-C drive but that's going to be expensive.
Buyer beware. There are USB-C drives that use the USB protocol, these are limited to USB 3.0 speeds with this computer. There are USB-C drives that use the Thunderbolt protocol, those with the USB-C port can be attached easily. Those with the USB-C captive cable cannot be attached easily. Check the specs before you buy.
A bit of a side note about the future of USB and Thunderbolt...
USB 3.2, which is not all that common and may not become common since USB4 came out so soon after USB 3.2, can reach 20 Gbps but only with the USB-C connector. This is the same speed as Thunderbolt 2, half the speed USB4 and Thunderbolt 3. USB4 is a protocol that allows the option to include support for the Thunderbolt protocol, making it backward compatible with Thunderbolt 3 and Thunderbolt 2 but not necessarily reaching the maximum speed of either. The USB4 protocol requires backward compatibility with previous USB versions but this can mean up to USB 3.2 speed, or less than USB 2.0 speed. To claim meeting the USB4 spec requires use of the USB-C connector, but doesn't require offering any higher speed than previous USB versions. Buyer beware again.
Oh, and use the right cable as not all ables are created equal.
A cable tested to meet the USB spec will have the USB icon on it and often a number indicating the speed it was tested to meet in Gbps. No number with the USB icon but with the stylized "SS" symbol usually means 5 Gbps tested speed, it might allow higher speeds, and it might not. A cable that has been tested to meet the Thunderbolt spec will have the Thunderbolt icon on it, this is pass/fail and not different speed tests like USB, the icon means it was tested at the highest data rate. No icon, or not a Thunderbolt or USB icon, does not mean it won't work but does mean it's not tested to meet the spec. A mini-DP cable without the Thunderbolt icon will fit a Thunderbolt 2 port but is not likely to work with any Thunderbolt 2 device. A USB-C cable with no USB or Thunderbolt icon will likely work at some speed, just perhaps not at the maximum speed the computer and device are capable. Buyer beware one more time.