I would like a plugin that allows only a specific rank to smoothly pass through iron bars while for lower ranks the bars are solid.
The client will not let the player pass through solid blocks. The best you could do for this is to send fake iron bars to the players of insufficient "rank". And you should also note what you're using to control ranks, or no one can help you.
https://mojang.com/2014/06/lets-talk-server-monetisation/ Just if you didin't know already, here's something new that Mojang decided. You're not allowed to have perks or items that make your server 'pay to win'. If donated for .. entrance for example, all players have donated the same price so that is allowed. Other types are not, though.
Ok well I use groupmanager and I guess I want you to make a plugin that makes fake iron bars i'll handle the rest.