I assume they'll want to go with a male replacement, and IF they go internal, this is my "short" list...
Tom Negovan: he's been there before, but he's basically their chief nightside reporter, and most frequent back-up on the late news. The broadcast has changed drastically since he left the noon news in 2008. Would he have the range and charisma to handle what the show's evolved into?
Ben Bradley: only anchors the 4pm, so this would be a cost-driven move, and he has a lot of versatility from his years on weekend morning at ABC 7. He does a lot with the investigative unit, though, and this could interfere with that schedule. It would also break their trend of set, cemented anchor pairings. He also rarely fills in at midday.
Dan Ponce: he's anchored the 4am block for several years, but like Ben, has very little time spent in that slot. The morning B-team has ascended the ratings ladder, and he's likely more valuable there than on the already top-rated noon hour (Midday trails 7 at 11am).
Tahman Bradley: weekend evening anchor, would bring much-needed diversity to the weekday male anchor bench. However, as political reporter, anchoring middays would likely leave one of the roles lacking.
Sean Lewis: weekend morning anchor for a decade, seemingly the most frequent fill-in for Steve. I can't think of many real drawbacks, so that why he's my predicted successor. That doesn't necessarily mean he's who I'd choose, though...
Patrick Elwood: he's surely the long shot. Of these men, he's the only one to never hold an anchor position at WGN, though spent many years as early morning/noon anchor on FOX32. Only in the last few years has he been used on the desk, mostly on weekends and at midday. That said, he's a warm presence, a local native, and damn solid anchor. If I was in charge, he'd be my pick.