This is a great idea that would get more people into playing more gameworlds. Seeing as the shops will soon have temporary sales, I don't see why the gameworlds couldn't have temporary bonus multipliers as well!
That element of randomness is a big thing that Tower's missing IMO. I'm really glad we're getting it back. Random plaza events and sales were a huge part of the replayability in GMT, since you always had a reason to stick around for the next sale or event. These bonuses would fit right in alongside them if you ask me.