Hmmm... not sure if I can help you on this or not.
I don't play GvG or PvP, so I can't give you any advise on that at all.
As for PvE, what are you going for? Health Degen, Condition, Interupts, or just fast casting?
Health Degen - Necros would be my 1st choice, they have lots of health degen skills that compliment Mesmers nicely
Condition - Necros or Rangers. Both can send off conditions from a far.
Interupts - Rangers are the next best for interupts, so massive interupting skills will endup filling your bar no problem
Fast Casting - Monks would be my first choice, then ritualists. Ritualists benefit greatly from Fast Casting (more so than monks), but monks are great healers and protectors (yes and smiting too).
Some people like to see a fast casting Nuker (but you need Prophecies to be a good nuker), but casting high level spells fast, seems to me like you'll run out of energy too quickly.
Assassins and Warriors are too much - in your face for my liking. Assassins do have nice teleportation techniques and healing (anyone is better than a mesmer in that). Warriors have awesome stances.
Builds - look around on each profession. There are plenty of builds, even with a mesmer. Just don't look at Mesmer/*, look at */mesmers too, it may give you ideas on what skills work well together.
In PvE, you can always change your 2nd profession and see what works for you. Once you get to a city (Sei... I can't remember the city name), you can change your 2nd profession as many times as you like. So experiment and have fun.
