A fairly standard dilemma - an RPG has a dozen races, a dozen professions and a party limit of six.
I would imagine that "what's the best.." questions are off topic, as they are in most SE site.
What if I propose a party and solicit feedback? I am looking for things like "you have 4 fighters and no magic users", or "humans make poor warriors".
Things like that are not opinion based; is it ok to ask?
Strategy questions are on topic as long as they are of sufficiently limited scope. A question just asking "Is this a viable party?", depending on the game, may be too broad. If possible, try to make it even more specific by describing what kind of playstyle you're going for, what difficulties you may already be having, etc.
I wouldn't say that questions about Party compositions are opinion based (at least for those RPGs I play) but they can be quite broad if not narrowed down to a specific problem you're facing.
Let's take WoW as an example (this applies to most (MMO)RPGs with party elements):
If you're asking a question like "What setup should we use to kill this WoW Raid boss?" without much further information it will most likely be closed since It shows no effort on your side and is quite broad.
If you however mention the exact problems you have with your group in that encounter the question becomes much more accurately answerable.
Note that the why is important here: why do you need a new group setup?
As a rule of thumb:
If you know why you need a new party (composition) we can help you. If you yourself don't know what you are struggling with we'd have to make a lot of assumptions in order to help and that's a close reason.
