That is a good question. It's kind of like "How to tell a good story", where there's no formula that will consistently give you good results, but there's a lot of pitfalls shared by bad examples.

I think that the best way to go about it is, mainly, to start out with a clear, sharp outline for the weapon in profile and in first person view. If you look at the MA5C and the SPNKr, you'll see that they are distinctive in both of those views and don't have an overabundance of details getting in the way.

Another principle should be to borrow from real-world designs and weapons technology so that you're not modeling fantasy weapons, but stay away from designs that are recognizable by the public at large. The P90 was fairly obscure when the M7 SMG debuted, same with the anti-materiel rifle that the UNSC's sniper rifle was based off of. However, Bungie notably abstained from basing their weapons on common designs like the M16 or the Kalashnikov.

Finally, new designs should be grounded in gameplay changes or in canon; take the BR-55 versus the DMR for example. The DMR functioned differently and was fielded by a different branc of the UNSC. The new rocket launcher would get a slightly warmer reception if it functioned differently, or if it was fielded by the Insurrectionists or a colonial militia.

: If you don't propose they ever create new weapons,

I don't.
Wait. I don't not propose that they shouldn't create new weapons.
Let's try that again; I am not automatically against 343i creating new weapons, though I haven't been impressed with their past designs.

: what if some or any of the new weapons became a tired brand/icon?
: What if the franchise needed some refreshing new weapons for any reason?

Most of the time, when I see an attempt to rebrand a franchise or bring new life to the material by changing the look, someone is either needlessly panicking or rearranging the deck chairs on the Titanic.

Fictional universes run out of stories to tell long before their design elements go stale.