It frankly wouldn't surprise me to see them essentially "reset" ilvl. There are multiple ways they could do it, including just dropping a digit and rounding where necessary. So top gear at the end of Heavensward would be maybe 28 or 30 ilvl max.

Or they could simply divide by some other number to reduce the ilvl.

That being said, they can continue with the pace they've been going for quite awhile before they have to consider resetting or restructuring the ilvl system. Between ARR and now, we've only gained ~140 ilvl I think. So if Heavensward ends at 280 or 300, that would still only put us at ~430 right before the next expansion after Stormblood.