I've always wanted to keep track of buff/debuff versatility, so this thread will help do that. There are currently 11 sources of which to acquire buffs and debuffs:
Class
World (only Vampire and Werewolf)
Guild (only Fighters, Mages, Psijic, and Undaunted)
Weapon
Armor
Alliance War
Monster Set
Weapon Set (*note: These have currently run the gambit of 1 armor set per skill. Either they will need to start doubling up, e.g. having 2 armor sets that affect, say, Uppercut, or include Ultimates or involve light/heavy attacking or be weapon-type specific--but that doesn't work with resto staves)
Craftable Set
Dropped (3) or (5)-Piece Set (which could further be divided into PvE and PvP)
Alchemy (Potions/Poisons)
The buff and debuff list is long, so I'll leave you to searching UESP for that information. But the ones desperately in need of more options are currently isolated in a single role's responsibility, a single applicable use, or only located in 5-Piece armor sets. I think ESO needs to bring value to more places by spreading out where these buffs and debuffs can be sourced. Maybe start by reworking unused sets and skills to meet this need. Then we can welcome crazy new set ideas like with "Glacial Guardian".
Buff/Debuff (suggestion):
Major Berserk (currently only on two MEDIUM 5-Piece sets and a Craftable Set that requires Break Free; needs ability source, though I honestly don't know where; maybe include with every weapon's ultimate since those are uncommonly used--except 2H and Bow in PvP and maybe Destro in PvE)
Minor Brittle (currently only on Ice Staves; needed on 5-Piece sets or included in Psijic's Elemental Weapon skill)
Major Brutality, Sorcery, Prophecy, Savagery (easily covered with skills and potions; all but uselessly included on Dreugh King Slayer, Rattlecage, Treasure Hunter, and Toothrow respectively, so those sets should either uniquely increase Damage/Crit by x% or we should see these easily-obtainable buffs as (3) bonuses on these sets)
Minor Brutality, Sorcery, Prophecy, Savagery (currently, this may give reason to include one of the four original Classes in the party, but what about when you don't have access to that Class? Again, a (3) bonus would fit nicely with these buffs or tie this into synergies for a smaller duration based on the activator's highest offensive stats. I don't know of anyone using poisons to obtain these--though I'm sure they exist)
Major Courage (currently on two LIGHT 5-Piece sets; needed more places than a healer's responsibility; rework Soul Magic and include this as a passive or supportive ultimate since that skill line gets little attention)
Minor Cowardice (currently on Nightblade Class, Poison, and 5-Piece LIGHT; needed on HEAVY and another reliable source like Vampire)
Major Cowardice (currently on Monster Set and 5-Piece HEAVY; needed on LIGHT or another conditional AoE source)
Major Endurance, Intellect, Fortitude (needed more places than potions; DK Class is only one with non-potion access to Major Endurance)
Minor Evasion (Warden's Class monopoly has got to end; the 10% is so minimal, just grant this for x time if you dodge roll 3 times in succession or brace for 5 seconds straight)
Minor Fortitude (Templar's Class monolopy has got to end; at least make this accessible via Alchemy or as a (3)-Piece bonus somewhere or tie it to overhealing with a Restoration Staff)
Minor/Major Heroism (needs more ability sources than only SnB and Warden respectively; create a new Undaunted Passive or Ultimate to grant this)
Minor Toughness (currently on Warden Class and 5-Piece HEAVY; needed on LIGHT for support and another source like Werewolf)
Minor Uncertainty (currently on 5-Piece LIGHT and Poisons; needed on 5-Piece HEAVY for support and in Alliance War like if it affects enemies that touch your Guard tether or damage the people tethered--so that ability can get some use!)
Major Vulnerability (the upcoming 5-Piece "Turning Tide" is good to avoid Kynmarcher's randomness, but it's still yet another 5-Piece; needs to be included on a Monster Set or something other than HEAVY or replace Mages Guild Might of the Guild--because who is casting a Mages Guild ability every 3 seconds for just Empower? Change to inflicts Major Vulnerability for 3 seconds after every 3 ability casts made within 30 seconds to your next target while in combat)
Other pseudo Buff/Debuffs and suggestions:
Healing Absorption (currently on a 5-Piece MEDIUM, Poison, and in Psijic Guild AoE skill; needs a more reliable single target application so as to not be easily avoided via dodge roll or a HEAVY support)
Unstoppable (currently only applicable to HEAVY armor users and via potion/poison--I'm not counting Immolator Charr--needs 5-Piece or even 3-Piece via PvP, like Potentates)
It's important that players have options for the build they choose. If you want to pull in enemies, great, you have 7 option for that (2 class, 1 guild, 2 (5)-Piece, 1 Monster Set, 1 Weapon Set). If you want Major Expendition, you--for some reason--have 20 options. If you're the wrong class, not progressed in the right guild, not Vampire or Werewolf, can only use 5-Piece armor sets, only have room for a 2-Piece Monster or Weapon Set, or can't craft the right potion or poison, you still deserve other options in how to build your character.
I hope this thread shines a light on the supportive combat elements needed. If older, unused and forgotten sets and skills were reworked to provide at least one of these, the game could blaze ahead with whatever wacky ideas it wants because the basics would be covered.
Thanks for reading. Let me know if I've missed anything.