Sprint just rolled out a Unlimited Kickstart promotion, which is unlimited talk/text/data for $25 per month, per line. No annual contracts. You can bring over your own compatible phone, or buy one from them. (Ex. Pre-Owned iPhone 7 at $12 a month.) Online orders only. New customers only. Requires port-in of your existing number.
Sprint has already been offering a year of free unlimited data for free. (I ended up paying about $3-$4 per line in taxes and fees.) But that requires a port-in from a postpaid carrier like Verizon, AT&T, or T-Mobile. If you are on a prepaid MVNO already, this may be a better deal since it has unlimited data. In addition, this offer lets you buy a phone from Sprint and they will finance it for you, so that you are paying $15 a month for a used iPhone 7 Plus for example. This could have you in a new phone and unlimited data for less than you are paying now for just cellular service. Both are opportunities to take advantage of the fact that Sprint really wants to up their numbers to improve their merger negotiating position.