Please forget about "having native GNU/Lunix" as a core goal: This is what Purism is trying to achieve with 2 Million Dollars, two years time, awkward hardware etc.

And a MediaTek SoC will not get you closer to this goal, rather the opposite.

You once stated that you are going to alter an existing design based on a Qualcomm SoC, which is absolutely reasonable.
If you have a choice, picking a design with a Snapdragon 650 (as in the Xperia X) or at least one with a Snapdragon 65x will significantly raise the probability of proper SailfishOS support.

As we already discussed here, a plain GNU/Linux without touch-capable applications is pretty much useless on a smartphone (and one cannot use the internal modem for calls and internet access, currently).

Please stay away from the idea of providing any software adaptations beyond boot-loader and maybe kernel, while trying to minimise the latter (kernel adaptations) as much as possible.
Do estimate these software efforts to be larger than the hardware redesign (people usually underestimate this by at least a magnitude, i.e. more than factor 10)!