Hmmm. According to the wiki, my hero(lvl 14) is approaching the max level(15) for the earliest task forces and the earliest trial. I don't wanna miss out on them, but I don't have anyone to do them with
EDIT: Wait, there's time travel in this game? Does it extend to task forces, I wonder? Need to do more reading.
You can access the earliest taskforce, Positron (part 1), at level 10. There's also the "Death From Below" trial that can be done at level 1. In most taskforces, all the enemies appear at the highest level for the TF.
The taskforce (free-to-play and premium players have access to) levels are:
Positron (part 1): 10-15
Positron (part 2): 11-16
Cavern of Transcendence Trial: 12-15 (Requires someone on the team to have unlocked the contact)
Synapse: 15-20
Sister Psyche: 20-25
Moonfire: 23-28
Terra Volta 1: 24-33 (Respec)
Citadel: 25-30
Hess: 25-30 (Requires someone on the team to have unlocked the contact)
Twilight's Son: 25-34 (No team needed to start)
Katie Hannon: 30-34 (Requires someone on the team to have unlocked the contact)
Manticore: 30-35
Mender Lazarus: 30-39 (No team needed to start)
Terra Volta 2: 34-44 (Respec)
Numina: 35-40
Imperious: 35-50 (Requires access to Midnighters' Club)
Abandoned Sewers Trial: 38-40
Eden Trial: 39-41
Dr. Quaterfield: 40-44
Sara Moore: 40-50
Justine Augustine:44-50
Faathim: 44-50
Terra Volta 3: 44-50 (Respec)
Lady Grey: 45-50
Dr. Khan: 45-50
Statesman: 45-50
Mender Silos: 46-50 (No team needed to start)
Cathedral of Pain Trial: 10-50 (Accessed by super group computer)
As for time travel, there's a zone called Ouroboros you can access at level 25 with time travel related story arcs and features the ability to play (or replay) story arcs you've out-leveled. Its also a convenient way of getting around the city. To get to Ouroboros, you need to use the Ouro portal power, which you can get from doing a story arc pertaining to time travel (such as the Faultline arcs or Ubelmann the Unknown (a lv 25-29 arc)) or hitchhiking on someone else's portal (I think you need to grab the exploration badge if you're using this method, which I think is on the metal bar over the pool).