The final conversion for getting my Patfinder/Midgard campaign moved over to 5e is finding some way to implement the Psion class, as presented in Dreamscarred Press' "Ultimate Psionics" (a great product, but very Pathfinder-y).
In this case, the player has specialized as a "Nomad" (teleportation, I mean "psychoportation" specialist), and in keeping with my "just get this working" ethic, here is how I'm handling his character. It's going to be a work in progress, and the player is willing (and eager) to work with me to get the feel right over the first few sessions.
First off, Psions are analogous to Wizards. They improve their powers through careful study and have "specialist schools".
I had already decided to use "Psionics as another flavor of Magic" in the PF version of this campaign. Simply put, "magic" derives from three sources - arcane (powered by the structure of the material plane, by way of ley lines, and wielded through ritual and pattern), divine (powered by the collective belief of trillions of souls, both alive and post-alive, and wielded through emotion), and psionic (powered by the creative energies of the astral "Sea of Possibilities", and wielded through mental discipline).
But for practical purposes, I'm using the "Point System" magic from the DMG, and adding "Nomad" as a school of magic with the following properties:
At 1st level, you gain "Psionic Focus", which is similar to "Concentration". It takes one action to become focused, and if you ever fail a Concentration check, you lose focus as well. While focused, you can "cast" cantrips at no point cost (Normally they cost 1 point), and can manifest any special abilities. Your Intelligence is your casting attribute.
At 2nd Level, you gain "Nomad's Step" - while focused, you can use your action to teleport within line of sight up to 15 feet away, +5 feet for each level above 2nd. You also gain "Catfall" as an always prepared Cantrip. (Catfall is a single-person Feather Fall limited to 20' + 10' per power point spent.) Note, replaces "Arcane Tradition" - not sure about gaining extra Feather Fall, but that seems to be a decent way to simulate the Nomad's growing control over space and time.
At 6th Level, you gain "Inconstant Position" - use your reaction to give any attack a 50% chance to miss you. You may use this power up to 1+(level-8)*2 times after a long rest.
I'll wing it from there.
I'm "reskinning" some of the spells to sound more "psionic-y", but the game effects will be the same.
I'm leaving Arcane Recovery in, though I need to work out how many points you can recover. It'll probably be something like "regain your caster level in points".
Lastly - since "Psionics" is another form of magic, it's treated as magic for any detection or resistance checks.
There are a lot of holes in this system, such as how many "spells" does a Psion get, and can they collect them like Wizards, or are they "hard coded" like a Sorcerer? Though "Sorcerer" matches the original vibe of "psionics is stuff you know how to do", I like the Psion class as an explorer of the Psionic arts, and though having to prepare a limited number of spells is possibly goofy in this case, I think it will work fine for play balance.