Things would really have to fall into place and be a critical need for them to go big on an OF, versus a more modest platoon/bench bat (ideally to replace Merrifield and/or Pache). They probably don't want to go over the last threshold either (though often the other team will pick up a lot of $. Except then you're giving up more talent too).
I don't know that Tucker would really cost that much - rumor season always starts with the biggest names being traded for 2-3 of a team's top 10 prospects and then ends with it being more like 1 at most (and then 2 from the Top 20 or 30). The year of control counts for something but I don't think you do that unless you're planning to extend him and have figured out what do about Nick.
I wouldn't say DD has gone big with the Phillies yet, really. He's been careful and judicious and only the Marsh deal involved a top prospect, for tons and tons of control. The way this team is playing does make you think big swing but if they are already going to win the division that one big deadline add isn't necessarily going to have much impact - especially not in a short playoff series.
I think it will be a lot like last year, a bullpen piece if the need is clear (right now it's not and they even let a guy who could have been the next Jeff Hoffman go for free) and the OF they didn't get last year. But a long way to go with Walker and how all the injuries and replacements play out.