I have three features I would like to see; one is more capable reminders similar to the current auto created mortgage reminders but more flexible, another is better ways to view a split transaction, the third is creating transaction from inside of loan accounts. They are related; the main reason the mortgage reminders bother me is that I find the current viewing and cross reference ability of splits to be very cumbersome.

1. Ability to create a recurring transaction that consists of a multiplier and an account balance on a certain date (an option for average balance over a month would be even nicer). For instance, a liability account, with interest is owed at an annual rate of 5%, compounded monthly. Similar to a mortgage. But payments can be flexible, so add the interest at the beginning of each month as one transaction. Any payments are separate transactions that reduce the balance.
   Creating a mortgage account with the original loan amount for the starting balance, an interest rate, and forcing the monthly payments to $0 simulates this with a mortgage reminder, if the pay from and interest field categories are the same. But that reminder creates a split transaction with three entries (one that says it is a split and has a balance of 0 in my interest category, one with a negative number in my interest category, and one with a positive number in the loan account).

2. I find splits very awkward. Extra step to edit, to view in the transaction history, and especially to cross reference. It can take x "View other side" clicks to find the cross referenced account I want for a transaction with x splits. An option to show the transaction as x separate transactions, each with their details and the ability to right click on any of them to "View other side", or to roll them back up into the current split interface would help a lot.
   Because of the split problem, my preferred way of tracking mortgage payments is to have two separate transactions, my bank account transfer to my mortgage to decrease the balance, and a transfer out to my interest expense to increase the balance. Because of the limitations on reminders, this is hard to do in an automated way.

3. I would just create the new interest transaction manually from within the loan account, but you can't create new transactions from inside a loan account. This restriction seems completely arbitrary, since I can "View other side" and create it from the expense side instead.

Summary:
Advanced reminders to create an automatic interest calculating transaction on any type of account, not tied to principal payments and splits.
Improved viewing of splits.
Allow new transactions to be created inside of loan accounts.