They'll do what they did for D:OS2, of course. What else would they do?
There will be a soft level cap per act, after which your XP gain will slow dramatically - so it will be quick and easy to get to the level cap for the act no matter how you play through it, but it will still be possible to get to the next level or two if you really scrape hard for XP - however, doing so won't help you MUCH, since there will be a similar soft cap in the next act.
