New planeswalkers will definitely have a huge impact on Modern format, and Ashiok is one of the best War of the Spark can offer.
It completely disarms fetchlands and Titanshift decks, and its minus ability also kills graveyards. The manabase also allows it to be played in either blue or black decks, which is a very smart design decision.
This means that Ashiok will be a great sideboard card, which can be deployed in case of need. The only scenario where it can be maindecked is in some sort of mill deck.
Lastly, note that Ashiok's minus ability allows to mill yourself, and exile opponent's graveyard, which is an elegant solution for Dredge mirrors.