Same as above. I don't want want anything too different to how it currently is. The grass isn't always greener, and the current version satisfies my competitive urge quite thoroughly. There needs to be a few extra players on the transfer market, the match engine can be a bit temperamental at times, and it would be lovely if there could be some kind of improvement to leagues (i.e., having group stages before knock-out stages) and improvements to prizes. Little things. But too many features will create confusion and annoyance (see SL2).
But I've said it before and I'll say it again, there needs to be a clear INSTRUCTION MANUAL for new players who join and don't know what they're doing. How can we expect people to stick around if they don't know which boots are best for certain conditions, which tactics are advantantateous, how to set up a team (and that they have to BUY CREDITS WITH ACTUAL MONEY to set up this team), as well as everything else about SL that we know and love. This is what SL has always lacked... an instruction manual that is flash bang in front of them as soon as they sign up, and an incentive to get them to stay upon joining. And the rest comes down to solid marketing... social media, paid adverts, screenshots of the game put about - the full works.
I'm happy to throw about some more ideas. I work for a technological firm and have been involved in the research/insight and marketing side of things for almost 4 years. (And have been a SL player since 2000... on and off). More than happy to help.