I would really want a 13 character gamertag, is there anyway to make this possible?

I think it's better for you to keep the 12, as it can present problems your account if you change it in 360 and you keep playing on xbox one, I wouldn't let myself play backwards since my xbox one password exceeded those of 360 , so I had to switch to
one of minus digits

I know that you're allowed to still use the old gamertag system if you go on your Xbox 360 console to change it. I know it's kind of a hassle to do that if you don't have it plugged it or have it stashed away but currently at the moment that's the only
way.

If you change the tag on a 360 you can still use the old system. Otherwise, through the new system there is no way to have a tag over 12 characters.

Yeah like i know people that have longer gamertags that are really creative. My gamertag used to only be Mega. Then i changed it and i wanted Mega back. Now with the new system i cant get my 13 character gamertag back. I can only make
it my ingame gamertag unfortunately

You cannot reset a Gamertag, this is not possible. The only option would be to make a new Gamertag if you really want to start over on a new tag.