Allowing the program to take several command line options as input, instead of just one.
+1 - in general, I'd like to see better automation possibilities via scripts / command line options. One example: Force MC to open with a certain zone selected, e.g. There: Zone 1 when using TRemote.
The ideal solution would be to introduce the concept of scripts - namely that MC could be started with a flag that tells it to read a certain text file. That text file contains lists of commands as per the current API, and MC executes them in turn. MC could either be opened and told to process the script file, or, if already open, executing MC again with the flag would tell it to run the list of commands in the current instance.
I would think this isn't too hard to do, as there is already an API allowing control of MC. We just need a way to execute a series of commands in a script.
In the future, this could be expanded by the inclusion into MC of some kind of scripting language, like Lua or TCL, both of which are often integrated into programs to give them scriptability. With that in place, one could write complex scripts with variables and conditions and loops and completely automate various MC functions.
That would make MC hugely modabble, without requiring the user is an experienced coder able to make plugins. I think you would see the amount of user-contributed modifications to MC would explode, because scripting langauges like the above are really easy to get in to and powerful to use.
Anyway, I'm not proposing that for MC 15 necessarily - but I think the basic ability to execute a sequence of API commands would be easy to add, and immediately useful.