Well I don't really know much about modding..But you would need a SOLID idea and somethign that your extremely yearning to do, nothing like "woa! what if i made a mod about *whatever*! you could this, that, and even this!" You would need a personality that would be fixed on this strongly so you woudldn't blow it off easily, because it's not something that within a month you will get "Okay it's comming along nicely", because it has almost been a year (i think more?) since 1.2 came out and they are still working on 1.3.
You would have to try and forsee what you would have to go through to create it, such as recruiting team members, making sure THEY are dedicated, make sure you know what this will have (revised ideas) and make sure the team members agree on basically what you want, you would have to commit to the project, so you would probably need to know how to code or model/animate, cuz they are usually the 2 biggies (codding>modeling in the cases i've seen).
You would also need to know how to work with the fun stuff and the boring/hard/frsutrating stuff, like in Flash, because if you don't, it could severly diminsih your dedication to the mod.
But, I am no develpoer, only using my expierinces on making a long-term flash movie. But in overall, make sure you know what you are getting yourself into.
And if you choose not to do a mod, I say learn Actionscript so you can make your own flash game