Commercially you may want to think about that as it won’t add much to your build costs but it will get you a wider market.
Batteries are by far and away the most expensive part of a drysuit heating system. Extending the life of an older battery is a good saving. Also many of us own the not quite 12V Light-for-Me heater batteries. 18AH but around 11V or lower. A change of battery is $$$$, well over a grand. Stomping up, say, $300 for a controller that would make the thing work makes great economic sense.
BTW I’ve two of those heater batteries.
Another feature could be a switching function for, say, 2 inputs and two outputs means being able to use two batteries on long dives without wet-swapping the E/O cables and also run two heating circuits — e.g. suit and gloves