Before the coup last year two planned new provinces made the news, including reaching international media. However it seems the plans to create the special administrative area around the new Bangkok airport (Nakhon Suvarnabhumi) as well as the unnamed province around Hua Hin, planned to be a gift to the 80th birthday of HM King Bhumipol, are both now shelved and won't surface anymore.
But not getting much prominence there seem to other new provinces in the making, some already mentioned in the navigational box for the provinces in the Thai Wikipedia:
* Chum Phae (ชุมแพ) or Phu Wiang (ภูเวียง), the northwestern part of Khon Kaen
* Mae Sot (แม่สอด), the five western districts of Tak at the border to Myanmar
* Thung Song (ทุ่งสง), the southwestern part of Nakhon Si Thammarat
* Bueng Kan (บึงกาฬ), the east of Nong Khai
* Phra Narai (พระนารายณ์), the east of Lopburi
* Na Thawi (นาทวี), the south of Songkhla
* Sawang Daen Din (สว่างแดนดิน), the northwest of Sakon Nakhon
* Fang (ฝาง), the north of Chiang Mai
I have no idea about the planning state of those provinces. And browsing the Royal Gazette there are entries talking about a province hall in Phon (พล, southern Khon Kaen), Lang Suan (หลังสวน, southern Chumphon) or Sikhio (สีคิ้ว, western Nakhon Ratchasima) - and I haven't searched systematically...