The signs are there. We could see it as soon as next season. Andre Villas-Boas' preferred 4-3-3 that won him a treble and Euro Cup with Porto. Tottenham now have a near perfect trio for AVB's vision. Sandro the destroyer, Moussa Dembélé the dribbler, and Lewis Holtby the passer. Watching Holtby play it's clear that he can occupy a center mid role in the 4-3-3. He's shown great defensive skills in his brief stint, making tackles and interceptions. The 4-3-3 typically involves two central mids taking turns moving forward and linking up play with the attackers, while the other stays deep and breaks up play. With Dembele and Holtby Spurs have a versatile duo linking up play in the middle of the field, one doing it with pace and strength and the other with creative passing.
Another thing AVB has done to convince he is planning a switch to a 4-3-3 is frequently playing Gylfi Siggurdson on the wing. I think he sees Gylfi's long term role as occupying the wing in the 4-3-3. Gylfi has shown flashes of brilliance on the wing, using his creativity to baffle defenders and create opportunities for his teammates. He also has the scoring ability that a player like Aaron Lennon lacks.