It could be an interesting thing to link SP and MP through a hub like this. In general though, I'd be fine with SP and MP being connected provided that the latter is completely optional, (i.e. all of the former can be done without touching the latter). The way ME3 is now is fine - you can get all the endings by playing just single player, or if you prefer, you can skip some single player stuff and make up for it by playing MP. The way ME3 was at release was not acceptable, in that MP was obligatory to get everything in SP.
I would, for example, be perfectly happy if I could earn money in MP that I could spend in SP, just as long as I could still earn enough money in SP to buy everything if I wanted to follow that path. I'd quite like to be able to unlock weapons, armour and customisation options in multiplayer. Provided, again, that there are ways to unlock them in single player.