Bearing in mind that I'm not a programmer or anything, I'll be happy if we get modding tools that are actually useful - modbuddy is absolutely horrible. It happily builds a mod, only for the games logs to helpfully point out that there's a syntax error near ")" or ",". As a layman, I don't understand why when I hit the build button my mods aren't checked against the game to make sure they'll actually work, and why errors aren't highlighted.
In Civ 5, there are colums in some of the database tables that aren't usable. It'd be helpful if they're removed in 6 so that it's clear what can and can't be changed.
I hope we're also able to add natural wonders through mods this time around.
Tangential note: world builder for Civ 5 is horrible, too - no copy and paste, no quickbar, no tooltips for things like promotions (even though, unlike modbuddy, it actually loads in all the info from all your mods) and do I even have to explain the nightmare of adding rivers?