I believe Related was planning the first phase, apart from DPI, would be a long "groundscraper" building.
https://www.instagram.com/p/B4DEf6IH8oH/
It doesn't look like these buildings are actually groundscrapers like OPO or Merch Mart, I think it's just more of a marketing gimmick to promote their large(er) floorplates.
EDIT: now that I see that image from CrazyCres, I'm even more sure that Related is courting Discover for the "Parkside HQ" (groundscraper). Admittedly, Discover isn't a tech company so they're probably not the intended tenant... but the promise of huge floorplates and "innovation" is tempting even for more traditional industries.
Sorry, but I can't see the Gateway Tower being the first phase... Discover will not fill up a building that large, leaving lots more space to fill. There's a lot of risk building that much space in an unproven South Loop submarket, far from Metra. Fulton Market is a much more established area and nobody's proposing supertalls or near-supertalls over there.
The 78 actually offers an interesting advantage, they can provide the initial tenants with basically unlimited surface parking which may help sweeten the deal as downtown recovers from Covid.