I don't expect big changes to the 2018 schedule. My impression is that Portland would be the most likely new venue. I'd assume it'd go into between Mid-Ohio and Pocono as that's one of the long breaks in the current schedule.
PIR has already a date for the 2018 VICS race in some documents. It'd be two weeks earlier than this week, like between the current Long Beach and Barber weeks. Barber would probably be a week later, especially if Long Beach retained its week from this year and was in the previous weekend to PIR.
However, March would badly need another race, especially if St. Pete keeps its current week. But the problem is that there are not so many venues able to host IndyCar in March. After St. Pete and Sebring, a Homestead race might be too much for Florida. Fontana isn't possible in March because of the NASCAR weekend. And I doubt IndyCar wants to leave the USA anymore after St. Pete. Extending the offseason and having St. Pete two weeks later might not be such a bad move since it'd eliminate the huge gap we've had here.
Overseas races would be a great way to shorten the offseason and kick off the season in February or even January. But I'm not sure about where IndyCar could race outside the USA. Surfers Paradise seems like the Australian place interested in hosting IndyCar but two street races in different months in one city doesn't sound very realistic. Brazil has failed to become a permanent fixture in the schedule. And Mexico City already has F1, WEC, and FE; IndyCar might be already too much.