Two Commander slots (at a stroke)
This has probably been discussed before but I can only find search results which touch on it without detailing this particular suggestion.
Why not have two separate 'slots' which a player chooses to play at the main game menu? The difference between them is that one is fixed in Solo mode and the other is fixed in Open mode. Neither, once begun, can be swapped. This would allow players to have another character identity and career path in the game with separate ship locations, so they can play a computer game if they so decide, rather than feel that they're bound to a virtual life (where the 'play' element can sometimes stray into unwanted 'poor paying second job' territory which is unavoidable). Sometimes, if you know you only have a short time to play, where you are in the game at present doesn't necessarily make expending that half hour or so very appealing.
Some might choose to go exploring in Solo but stick around inhabited space with their Open character, involving themselves in PP activities. Others might like to go exploring in Open, hoping to bump into a fellow space traveller somewhere in the black, whilst trading in Solo as they want to avoid meeting aggressive human players. Others might use their Solo character to try out newly introduced game mechanics, reserving their Open character as their primary game playing choice. Everyone will have their own favourite way to separate the two.
I see no reason why this concept should upset the apple cart in any way whatsoever. If each 'slot character' had its own credit balance and ship ownership then they are divorced from each other. The Solo character could only affect the galaxy as any player currently playing in Solo does now. The Open character slot could mean that more people who currently play exclusively in Solo might be tempted into the Open arena. If anyone chooses to play in Open then they have a greater commitment to whatever role they are engaged in, as they can't just switch over to Solo when things get too hairy, yet carry on seconds later from the same point with the same ship, balance, cargo, etc.. The concept of two slots per game licence holder would be like doubling the player base in one sense, but the maximum number of concurrent players would not change.
If this was to to introduced then there are two things which would need to be done beforehand. Firstly, the plan should be announced several weeks prior, so that we can position our current characters where we want them in the game universe. Secondly, the main game menu should make it clear when starting up the game after the upgrade that the choice of current character becoming Solo or Open is binding and cannot be reversed. As each licence holder has only one log in id then there should be no possibility of each 'slot character' being played at the same time (unless FD decide that it is acceptable).
If it was decided to introduce this two-slot feature I'm undecided how I'd use my two slots. I do know that I'd keep my current character in Open, and having a new character to play with beginning as a noob would be quite fun.