Which alt should i pick?
My first choice would be none. Stick to a main. Get a legendary. Have lots of gold. If one the other hand, you are opposed to gold and BiS gear then I recommend Guardian. Very different from the Mesmer and one of the best classes in the game.
Enjoy being broke.
Anything I post is just the opinion of a very vocal minority of 1.
It all depends on what you personally prefer, since enjoying a class and understanding it intuitively is the only real consideration that matters. With that said, Elementalist can do a ton of damage and has great access to support if you use a staff, and conjured weapons are always useful in PVE. You could even run a signet build that lets you retain the passive benefits of equipped signets even after you use them, which is a good way to stay alive (at least from my experience). Engineer is also a solid choice because you can choose which kits you want to specialize in, and there are tons of support options. Engineers are excellent healers and supporters and have some of the best bunker builds in PVP.
Mesmers are a bit harder to use, but if you like the Mesmer’s playstyle then go for it. However, they play rather differently from the other classes and aren’t intuitive for a lot of people. It takes more planning to come up with a good Mesmer build that uses skills and traits that synergize well than most builds with most other classes. I would highly recommend reading about different kinds of builds in the profession subforums to get an idea of what each class is capable of and to figure out what tactics appeal to you the most.
if you have a mesmer and an engi, then you still have room to create the other 3 professions that you’re interested in. every class is good in PvP and they can all be built for the same kinds of things. If you like mesmer and engi and are fluid with those though, I would recommend a thief and an elementalist. Those are the 4 most complex classes to excel with and if your fingers are used to the enormous amount of button mashing, finger breaking, hand gemnastics associated with those two classes, then elementalist and thief will be right up your ally. they are all very different classes but with their aggressive playstyle based on cycling through their abilities quickly, your finger’s muscle memory will feel right at home with these. Ranger is a bit this way as well as controlling your pet can be a pain (though very rewarding) and is a ton of finger snapping goodness in itself. After using mesmer and engi, the warrior, necro, and guardian will seem like the most mind numbing playstyle ever tbh. not to offend anyone but it’s the truth.