Welcome to the forum and being a gun owner
I am not a lawyer, am not from illinois, and don't know the wording of their proposed legislation. I am sure you can get information throught the NRA-ILA website that will give you more detail of the potential changes in your state.
I have read New York State's new laws and can tell you that they did impose a 7 round magazine limit. The articles I read on the matter say that those with 10 round magazines (which I believe they were limited too already) are allowed to keep them but can be charged with a misdemeanor if they are found in possesion of magazine with more than 7 rounds in it. They are not allowing magazines with a capacity higher than 10 to be grandfathered and residents of new york would not want to be found in possesion of them. Obviously what happens in New York isn't necessarily what happens in Illinois, but I would say the 2 states have similar minded politicians.
One would hope that items that were previously legal in your state would be grandfathered into new legislation, but obviously there is no gaurantee. If a bill should be passed after you have purchased a glock I would not rush to do anything with it or the magazines as I expect class action suits and alot of other measures will be used to get these ridiculous laws overturned or amended. You will also find that most manufacturers are very good about making products that will be legal in your state and that you should eventually be able to get compliant magazines if it comes down to that. States like california are the reason manufacturers make so many models of basically the same pistol. They have to make a compliant model for that state and the standard model for all the others.
In the meantime, write or call your representatives and tell them you are not in support of any new restrictive legislation!