Just some good ones I've seem come up when discussing the topic with other players:
1. Artificer's Guild
A group dedicated to uncovering the secrets of ancient civilizations, most notably the ayleids and the dwemer. This guild advances its members as they collect valuable artifacts from dwemer and ayleid ruins, and additional advancements may come with solving ancient dwemer puzzles or re-activating/repairing dwemer machinery.
Abilities:
The artificer's guild skill line would be a stamina-based healing and buffing line - an alternative to the magicka-based restoration staff or restoring light trees currently available for healers.
Utilizing ancient dwemer artifacts, the wielder is able to regenerate the health of allies or buff their weapons and armor. The line could also include an ability to summon a combat pet - perhaps a dwemer spider or sphere - to help the player fight, similar to the pets used by sorcerors.
2. The Way of the Voice
Run by the greybeards, followers of the way of the voice learn to utilize an ancient form of magic known as the thu'um, speaking in the dragon tongue to affect change in the world. These followers are not dragonborn, and do not have the powerful abilities that come with such birthright, but over years of study are able to use the thu'um in more localized ways.
Abilities:
The way of the voice would provide players with a set of voice abilities similar to the shouts used in skyrim - although obviously less powerful and less advanced, we're not dragonborn nor should we be. Personally I would make this skill line utilize the ultimate resource for each ability, rather than stamina or magicka - basically a guild tree that gives you 5 ultimate abilities to choose from, but of course you can only have one on your skill bar (two if you're swapping weapon sets) at a time.
Basing the abilities on ultimate would make them seem a little more epic, and give them the balancing factor to be a little more powerful than your average spell or weapon attack.
3. Pilgrim of the Divines
A skill line granted by making a pilgrimmage to the temples of the divines in each faction zone and/or cyrodiil. Essentially this skill line would focus on monk-like unarmed attacks and holy blessings bestowed upon the player by one of the eight divines.
Abilities:
At least one attack based on unarmed combat would be nice, but primarily the tree should focus on auras or group buffs - one for each of the eight divines in this time period.
4. Daedric Cults
Essentially the opposite of the pilgrim of the divines line - this skill tree would allow the player access to buffs/auras based on the more notable daedric lords (malacath, molag bal, azurah, etc.)
PS4 / NA
M'asad - Khajiit Nightblade - Healer
Pakhet - Khajiit Dragonknight - Tank
Raksha - Khajiit Sorcerer - Stamina DPS
Bastet - Khajiit Templar - Healer
Leonin - Khajiit Warden - Tank