This is going to be a running compilation of ALL skill lines wanted by the community
it will be updated every now and again with new guilds posted in the comments or found on the fourms

1) Temple/divines (a healing skill line, missions styles follow the temple arc from morrowind.
Actives - channel heals and instacast heals
Passives - resistance to elements and return heals

2) Deadric ( you do quest for deadric lords)
Actives - self buffs and point/aoe debuffs
Passives- added effects when using deadric skills and stat boosts

3) Brotherhood ( tgt kill missions and dead drop missions with a main quest when you finish all dead drops/tgt kill in a zone)
Actives- stealth and nuke skills extremely high H/M/S cost
Passives- crit boost, stealth boost, and a flanking additive

4) Thieves Guild (grab missions and item farm missions, Story missions after a gold value is reached)
Actives- out of combat pick pocket(small stuff NPC only) and Stealth/Speed boosting skills
Passives- Stealth boost, % gold gain boost, grants special venders.


5) Merchants Guild (buying & selling from vendors, buying & selling in guild store

passives
increase in sell prices to merchants 5/10/15%
Reduction of guild store listing fees by 25/50/75%
Increase inventory space by 25/50/75/100 slots
Increase bank space by 25/50/75/100 slots
*idea taken from @Shimizu‌
Please if you have any more ideas add them and try to follow this format I will be making a edits to this for other ideas you all post so its at the top.