An energy SMG has been requested before. This weapon class could use one that uses energy ammo. But how to make one that fits the theme of energy weapons, is balanced and doesn`t "step on the foot" of other weapons by working exactly the same?
So lets start with it being energy themed. Energy weapons have the unique trait of low projectile drop, high damage per bullet and a charge up before they reach full efficiency. (for the triple take it`s the choke charging). But charge up on a SMG? You want to use this thing up close so any delay is a big problem. A good way to keep the theme and make it good close would be making it have a pretty large recoil and bullet spread for the first few shots (4-6) and after that it would "charge" into almost perfect accuracy. This way it remains pretty solid at close range but it`s bad at longer distances, essentially making you waste the first few shots. So it`s a good short range weapon – what it`s supposed to be.
To keep the thing balanced and considerably different from other energy weapons it should have a pretty small mag. Havoc and Devotion have huge mags allowing to spray a lot. An energy SMG should have a really small mag with a lot of damage each bullet. Compared to an R99 that has 18 bullets x 12 dmg it could be reversed having 12 bullets x 18 dmg with the fire rate altered for similar dps. 18 damage per shot might be kinda extreme for an SMG but what it does it makes it not only good up close but also able to deal some poke damage further after you waste some shots to the initial "charge". This way it makes some use of the low bullet drop.
Of course it might be hard to keep the thing balanced in the current game where energy weapons are so common without nerfing the spawn rates of some of them (i`m looking at you charge rifle). With energy ammo being less common i think an energy SMG working like described above could be balanced and with a playstyle considerbly different form the other SMGs it would be an interesting addition.
Note i didn`t play Titafall games so i don`t know if any weapons from that universe would work in a similar manner. I heard that there was something called the Volt but i got no idea how it worked.
If you liked the suggestion let me know. I got a few more ideas i can post later.