I take no oath, but I believe both sentences are grammatically/syntactically correct.
There's a difference in meaning, though.
What I have proposed vs 2.
What I am proposing
connote a proposal made at an earlier point than the time of speaking/writing
a proposal made now, as we speak/write.
That is, of course, at a literal interpretation of the sentences. In practice, I 'd say the latter might be used to connote the former, but not vice-versa.
we should have begun to think seriously about a thorough reform in the legal system
we begin to think seriously about a thorough reform in the legal system
First sentence is not really a proposal/suggestion, in the literal sense of the word. You are rather underlining the importance of thinking seriously about a thorough reform in the legal system, by stressing that it should actually have already begun. No further delays, fellows!
In the second sentence you are indeed suggesting that these consideration should begin.
Let's hope this wasn't too vague...