Matua devotees state that in the orthodox Vaiṣṇava scriptures it is written that Chaitanya will be reincarnated and will appear again in the land of Iśān (the cardinal direction of north-east). This land is interpreted as Bangladesh, where Harichand Thakur was born. Is this claim justified in any scripture of the Gaudiya Vaishnavas?
Yes, in the chapter 17 of the Madhya Lila of the Chaitanya Bhagavatam, Mahaprabhu promises His devotees that He shall reincarnate twice in the near future:
The Lord could not bear the pain His devotees were suffering. Smiling pleasantly to alleviate their distress the Lord assured them, "why do you unnecessarily vex yourselves? I am always present with you. It is wrong of you to think that once I have taken sannyasa I will leave all of you and go far away. Not for a moment can I leave you. All of you are My eternal associates. Not just for this birth, but birth after birth. Those of you who are associated with Me in this incarnation will eternally remain with Me in the mellow of congregational chanting of the Holy name. In every millenium I advent and in every one of My immumerable incarnations you have participated in My pastimes. With this advent I have come with two incarnations, one is the congregational chanting and the other is My transcendental deity form full of bliss. In both these incarnations you must participate in My pastimes with full vigour and joy, chanting with Me in ecstacy. I am taking sannyasa only to educate humanity, therefore please allay your fears and worries."
Sacidevi spoke on unable to calm herself the Lord still remained silently listening. She felt drained out, rampaged by the storm of grief, food and sleep were to her now a curse. The Lord seeing His mother's condition, revealed to her some secrets that might pacify her. He said, "Mother, please calm yourself and listen, I have been your son since many births. You were once Prishin and I was your son. Again you became Aditi in another life and you were residing then in the heavenly planets and I became your son Vamana. Then once again I became your son Kapila when you were Devahuti. When you were Kaushalya I was your son Rama. Another time you were Devaki in Mathura imprisoned in the dungeons of the cruel King Kamsa, and I became your son Krishna. In this present incarnation I will be your son twice. You will be the mother of My incarnation as the "deity form", and also the mother of My incarnation as the "Holy name". Mother, like this you have had Me as your son eternally, I can never leave you mother. I revealed this to you very confidentilally, now mother please allay your fears and grief.Sacidevi on hearing this esoteric narration was somewhat calmed.
As a sidenote, I would like to add that the Vaishnavas of the Mahanam Sampradaya (an offshoot of the Gaudiya Vaishnava Sampradaya) interpret these verses to be referring to the birth of the twentieth century Vaishnava saint, Prabhu Jagatbandhu, who is considered by His followers to be a reincarnation of Mahaprabhu Himself (which I discuss here). However, Prabhu Jagatbandhu was born in Murshidabad, West Bengal, India rather than Bangladesh (though He spent most of the time in the Bangladeshi city of Faridpur). Nevertheless, there is no reference to land of Iśān in the Chaitanya Bhagavatam.