There is tldr

1)PvE
Replace unused morphs with PvE ONLY boosting morphs (deal extra dmg to monsters/boss) in order to bring less strong skill lines on the same Lv as the meta

2)Add one more ability per weapon skill line. A spammable without OP effects

3)Either add new sets that offer 650 W/S dmg when using a certain weapon, or buff the existing sets. Point 3 and point 2 may help introduce new builds. Bow/bow, SnB/SnB and so forth.
I am sure that if people had one more damaging weapon skill and a strong set to improve their weapon skills they would replace their hundings and go for a Sword Singer 2h/2h.

4)Elemental Weapons
Lets pretend that point 2 happens and we get 6 instead of 5 skills per weapon.
This new skill line could have 2 skills from 2h 2 skills from DW 2 skills from Bow (SnB can work as it currently does for magika builds)
These skills would use magika and deal magic dmg. One morph would be PvP and the other would boost dmg against monsters and bosses.
Depenting on the class of the wielder we could have lightning weapons, flame weapons, holy weapons, shadow weapons, frost weapons.

5)Add some short of orb/book magic weapon.

6)Add 2 more stamina morphs per class

Btw they added weapon ulties(which was wrong). They can add another skill per weapon.

Zos. Build a Dlc around magic weapons. Introduce more PvP locations around that theme... relic weapon hunting... I dont know.

The thieves guild and Dark brotherhood skill lines and instanced activities were a waste of resource/time. Personally I got bored with them within a week. Others will claim that doing those things is the most important aspect of Eso. I am waiting for these comments.
Active weapon abilities should be the way to move forward the game. And new classes.
Not skill lines full of useless passives and boring solo instances.
Looking back at thieves guild and Dark Brotherhood I am thinking "what the hell....."
These DLCs just needed a story quest line.
No new treelines. No new instances. Far out.. so much energy wasted


Tldr
1)PvE morphs on unused weapon ability morphs
2)One more dmg spammable per weapon line. No crazy effects
3)Buff sets that increase W/S dmg when using certain weapon skills.
4)Elemental Weapon skill line with unique aura per class and access to 2 magica weapon type skills except SnB
5)Magica weapon.
6)Stamina class morphs