For folks seeking a step-through donor pedal bicycle frame to convert to an ebike with a DIY motor kit, the Reddit City Bikes spreadsheet has a column indicating where a step-through frame is available together with price, type of drivetrain, and web link.
Common features of ebikes used in urban bikeshare systems such as the
Smoovengo E-Bike (Paris), Uber JUMP (Washington, DC, & San Francisco), Bewegen Pedelec (Baltimore), and
BCycle Dash+ (designed by Trek, coming in 2018), are a step-through frame, 26" wheels, 3 or 7 speed IGH, Class 1 pedelec, 250w front hub or 350w mid-drive motor, and rollerbrakes. Dock based systems recharge off the bikeshare dock vs dockless systems like JUMP incorporate a GPS locator chip and require you lock up the ebike with a provided U-lock and a maintenance guy either swaps out the battery or recharges it at a hub collection point every 2-3 days.