Save the other arms for other potential ARMS characters.
Or, if custom moves return, some extra arms as customs would work nicely.
Custom moves shouldn't exist outside of Mii Fighters and reserving every non-default arm for a potential future ARMS character is absurd. We're probably never getting one, and we certainly aren't going to get enough to justify having a hard limit on who can have what for the sake of it. The game has dozens of weapons, there's nothing offensive about giving "each" representative six with three as one-offs.
I reckon he should stick with his signature ARMS. Those three plus the whole ranged gimmick are more than enough to make a standout fighter, and anything more could get cluttered.
There's a reason Dhalsim has fire (/yoga magic) and Necro has electricity. Choreographically you definitely could get a whole moveset out of just the default set, but it would mechanically be very limited. Having specials that do something unusual also gives him room to have more unusual standard attacks (like Megaman's), because you aren't using one of only three weapons he has on a special.
Both of his extra arms are projectiles, you could reasonably use that as a justification for making those specials, but if you give, say, Tribolt a dedicated special for "this represents this weapon's function," it would be weird to also put it in his standard moveset. There is simply not enough in his normal loadout to justify not using any other arms.
I realize that I'm not being very clear, but hopefully we've all played the game enough to have an intuitive understanding for why, I don't know, Yoshi has three egg-based specials but they don't appear as standard attacks, or how Link uses tools for three of his specials but then sword/body for his standards. I'm appealing for the same thing here, use the special moves for special/one-off abilities and let his core features be used more fluidly throughout his standard moveset (with the charging mechanic being the core feature that's still a special).
Firmly disagree. In the end, there’s no difference between a long punch and a laser sword and what have you.
I have no idea how you can say this if you've looked at framedata before (or even just the choreography, honestly). There absolutely should be a difference between a long, sweeping hitbox like the beam sword mostly gets you and what Dhalsim's punches would look like if he was in the game. The timing, movement, and proportions of different hitboxes are the core mechanic of fighting games as a genre, and saying there's no difference between a punch and a sword swing is akin to saying that almost every standard attack in the game is identical.