(edited by moonboi.1764)
A profession to complement a power mesmer
In PvP, playing shatter mesmer + burst thief is a very fun combo to play together. You can set up synchronized burst for deadly effects.
On the other hand, you may have more success playing on the contrary a team-fight oriented spec (rev/guardian/ele/necro etc…) because having 2 roamers in the current meta means you will get assassinated in team fights.
A good compromise is probably a power rev since it can both be a great roamer and a great team fighter.
For open-world PvE, anything works just fine really. If your fiancé goes for chronomancer, he/she can bring alacrity/quickness support (since chronomancer/mesmer is really a bad PvE choice if not for the support) so any good DPSer would have good synergy (DPS ele for example).
Finally if you want to go for group PvE content, especially raid, it will mostly depend on what the other team mates bring.
Hope it helps… somewhat
It does a lot. Thank you very much. I was leaning towards necromancer a bit or ele.