The vast majority of existing 1.6 plugins should be compatible with 1.8 almost out of the box. Literally all you should have to do is edit one line of code to identify the plugin as 1.8 compatible. I've done that on my plugins and porting them to 1.8 literally takes 60 seconds if only the bare minimum is done, and they work just fine. (There will be plugins where that isn't the case, but I have yet to run into any difficulties yet in porting my own plugins. )
That said, MyBB 1.8 does add some new setting types that are very useful, such as the ability to create forum select settings or usergroup select settings. (No more "enter forum ID" or "enter group IDs" jargon. Kudos to the MyBB team for implementing that, BTW. ) MyBB 1.8 has added a few new hooks as well, and has implemented certain features such as the default avatar functionality that may integrate differently in 1.8 plugins than in 1.6 based plugins.
Personally, I'd recommend just going ahead and developing them for 1.8. MYBB 1.6 won't be the main focus for much longer, and if there is really a lot of demand for the plugins, it should be a fairly straightforward task to backport them to 1.6 later, if necessary. It's best to stay bleeding edge and to focus on what's current in most cases.
Good luck on your plugin development.