$next_charge is slippery

No that solution would limit the flexibility of this module. How would this handle giving one members a free week when their intervals are normally a month.

I think the solution here is to add an extended field, which keeps track of the time extended. Then when renewed it would add the regular interval minus what ever the extended field contains, reseting the extended field for next renewal.

I can't really see how adding three fields - 'start_date', 'recur_count' & 'freebie' is a better solution to just adding the one field for 'extended' and using that to keeping track of how long a subscription has been extended (for whatever reason, e.g. freebie, failed payment) so the next renewal dates can be accurately calculated to include any extensions that occur.

Recurring fees and Order objects already contain a created field so that could be used to determine the start date if you want to find that out.

If you want to make the first payment prorata to get the recurring payments happening on the same day each month you could use the available hooks, like the following:

// set the first renewal date
function hook_recurring_fee_user_create(&$fee) {
  $fee->next_charge = // calculate and set date to the first of the next month
}

// you can also check that the renew is going to happen correctly each month.
function hook_recurring_renewal_pending(&$order, &$fee) {
  // double check that the next_charge has been correctly calculated and adjust if neccessary
  $fee->next_charge;
}

You will also need to alter the order to change the amount on the product to be a prorata amount.

Here is a quick simple fix to this issue.

Committed my patch above.