There's a couple of ways but your best options are the Proposal Boards and Feedback Forms. Make sure you participate in the Form every month but also share your ideas on the Proposal Boards so other players can state what they think!
Not the preferred method as support tickets are generally there if you need assistance, they will see if they can forward your suggestion but might steer you into submitting a proposal or feedback form-
I think the best option would be to use Tibia.com forums, there is a board called "Proposals". To reach it press "Forum" on the main site, then "Community Boards" and there's section "Proposals".
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