Will you buy if MEIZU release a Tablet?

Y E S ! ! !

With the increased size of smartphones, I believe tablet is no more required

They could do the Meizu M3 Tab

...and make an m-pen (stylus pen) that functions as a phone.
Imagine working with just a tablet and a stylus pen that functions as a phone, and which is compatible with a smartwatch = no need for a chunky smartphone.
Walking around with a phone the size of a marker or just lock it in a tablet, or its case, would be so much easier.

Everybody is free to believe what they want. I believe the size of your screen depends on your workflow. I see a a big future for smartwatches and tablets in combination with a (modular) pen-phone. Also camera's, bluetooth speakers and what not, are all internet connected and have touch screens, WiFi, cloud, IoT...
Now imagine connecting your pen-phone to those devices with a (high res.) touch screen, ...now your smartphone is reduced to a mini tablet. So we are back to the size of your screen depending on your workflow.

...If you could imagine a modular stylus pen with phone speakers, interchangeable parts, mouse-click, connected to a smartwatch, etc... yeah, that would be great!

I think they will, in their mockup there is sometimes a tablet. It will be a signal? : D

Your idea is good. But it is far from reality...
In everyday life we need something which is easy to carry (preferably in trouser pocket) and can satisfy our workflow requirement. A tablet may be useful for a office executive... but imagine when the same person is out for a vacation he has to carry the entire tablet. Either he/she has to buy both ori If he/she carries a smartphone of reasonable size which can do everything, obviously a smartphone will be preferred.

But remember what we do in a PC/Laptop, the same can not be done in a tablet device with perfection. Teblet devices are there to complement our works and if same can be achieved through a smartphone, a smartphone will be always preferred.

Smartpen, smartwatch - they are accessories and can be used along with smartphone or tablet. They are compact, they replaces are regular accessories and can be used to get benifitted.