Okay, so I think we can ALL agree that one of the reasons why SL2 never worked out was because it was far too complicated. It was too fiddly, and there were far too many things to think about (e.g., stadiums, fans, equipment... I can't think of any more features because it has been years since I logged into it). The reason why SL1 has excelled and have continued to excel is because of its simplicity. Simplicity is often best, and it doesn't take very long to master the game.
Which brings me to my first improvement/suggestion: The game IS easy to learn and master, and it does not take long to pick things up. New users would not know this... and this is why accessible training/tips/guidance is the VERY FIRST POINT to consider. New users need to feel that they can click on a link and receive immediate instructions. You wouldn't play Monopoly for the first time without reading the instructions, would you? They need pointers about what they should do first; setting up tactics (what tactics beat what), training (inc. what each of the training options improve), private leagues, transfers, etc --- everything that is important to playing the game. They'll need to know that in order to stand a chance, it is recommended that they purchase credits. I can imagine that some people would recognise that they need to buy credits to compete, and think: "no, I'm not spending any money!!" and not log back in again. This needs to be clarified sooner... perhaps there could be a credit offer for new users? We want SL1 to grow and improve and thrive, so this is the No.1 way to attract and potentially keep new users. This needs to be done before we go all out on advertising.
At the same time, if you have played the game for years and years, its simplicity may start to get a bit boring. So we also need something extra for old users... something simple perhaps, but just SOMETHING to provide users with something - anything - new to focus on. What this is, I don't know. A new feature? New tactics? (although, I remember pressing didn't work when it was first introduced). More advanced tactics? More attributes? A new pyramid system (with one top league?) More league competitions (a champions league?) One of the reasons why many users continue to stick around is because of the Friendly League, so it seems obvious to me that the game would benefit from exciting new leagues for everyone to take part in.
So in summary, we need to appease new users by showing off its simplicity via detailed instructions, and we need to appease old users by introducing something new that makes the game a bit more interesting/isn't complex/that new users can also understand. The game does not, in my opinion, need renovation. It just needs a couple of tweaks and maybe 1/2 new feature(s) to play around with.