The only way to make Open more relevant in BGS and in Power Play, is to add influential game play that requires interaction with other commanders.
Bingo
Unless there is relevant PvP related impact the open/solo debate will remain meaningless. Open only will not change anything significantly in the absence of a number of other changes including the above.
3 different platforms: cross platform play will need to be developed.
Timezones: Blockade for 16 out of 24 hours? the Aussies will sneak in while you sleep! Cant see any real solution to this.
16 player instance limit: This would need to be expanded significantly (a single instance ? good luck with that)
Friendslist bias for matchmaking: This would have to be changed.
Location bias for matchmaking: Overcoming the instancing and lag issues (plus server loads)
The limited amount of time in SC needed to get anywhere and do anything: No real solution to this one. windows for opportunity are few and far between.
Even with open only, without some other incentive, meaning or funnelling, player interaction will remain rare under the current design.