1- Challengers should also have qualifications, and you shouldn't be able to enter main draw of a challenger unless your are ranked inside top 200;
2- Challengers should also have doubles competition;
3- You should get additional points for getting through qualifications on any tournament;
4- You should be forbiden to play challengers if you are ranked inside top 150;
5- To enter qualies of world series or masters series tournaments you should be ranked inside top 150, and for grand slams inside top 200;
6- You should be obligated to play all grand slams if you are ranked inside top 100, and all masters series events if you are ranked inside top 50 - if you decide to play another event at the time of one of those, you should (for example) be penalised by your points earned on that event cut in half;
7- You should be forbiden to play the smallest world series events (smaller than half a million in prize money) if you are ranked inside top 10;
8- Players should get injured once in a while - for example, if your combined match time goes beyond 20 hours without a rest between tournaments (combining singles and doubles), you should get injured and be forced not to play in the next 3 events (to recover) - after coming back from injury your stats should be slightly lower;
9- Your form should also vary, meaning you should find your stats getting temporarily lower during about 5 random events each season;
10- Switching between surfaces should lower your stats, cause it takes time to adjust to new conditions;
11- Taking too long a break between tournaments (say, 4 weeks or more), should lower your stats due to inactivity;
12- CPU players should be more inteligent - when I learn how to play them, I almost never lose again.
So my main suggestion is basically to make the game harder, cause while it's like this, it's only hard in the beginning, but once you start winning your stats never fall and you're destined never to drop in rankings. It's also unreasonable to be able play how much you want, whatever you want, cause that way you can have a top 20 ranking just by playing challengers. Furthermore, after playing only one season on Incredible level, I'm number 1 in singles rankings, I've won 2 grand slams, world championships plus some smaller events and participated in doubles world championships.