Write a better wiki-plugin, if you look at the existing one for a syntax and code template in lib/wiki-plugins they are easy to complete to more your liking.
To code some symbols to work as super and sub script you'll need to modify regexp's in tikilib.php, that is much more work.
If you need help with wiki-plugin writing, contact me I have services available.
p.s. Allowing groups of people to edit HTML code in wiki pages is asking for your tiki to be hacked. HTML in wiki pages is not secure, and it is possible to gain control of your site.