The suggestions box on playerdex is the official medium for submitting suggestions.
Although the suggestion box on playerdex is officially accepted as a suggestions depository over the forums, the forums promote a more active discussion on them, if community/staff feedback is sought out; until the suggestion box is more wholly developed. The suggestion box is still preferable in the sense that they are less likely to be clouded by non-suggestion threads, however.
Also, as a general protip, it's highly encouraged that you perform a forums search, to know what has already been suggested and assessed.