What you're proposing has already been done unfortunately.
Mysql was sold to Sun (The company behind java) The creator of mysql wasn't happy about the way Sun is monetizing mysql.
There's a commercial version of mysql and the creator about mysql wasn't about it. There are other issues, for instance you can't upgrade to the latest version of mysql if you're through YUM or APT, you have to use RPM or if you're on debian based on you'll have big troubles upgrading to the latest version of mysql.
So what did the creator of mysql do about it? He created MariaDB, fully open source project, full support to MYSQL, used by millions today. The creator of mysql has the right to use 60% of his original mysql code, which what he did, MariaDB is a lot better than MYSQL. I'll throw some links about MARIADB, check them out.
If you want to create a new database, don't reinvent the wheel, take a look at MariaDB source code first, see what can be improved, do some performance tests on MariaDB and MYSQL, compare the results, if you're confident that you can create something better than what MariaDb has to offer then go for it, iif not then it's not worth the time and effort.