Start with 5e, add 5e Advanced rules and 7.7 mechanics (like interrupts and partial success). Remove classes, replace paths with Power Sources.

Result:

• Simple skill proficiencies that should be familiar to any 5e veteran
• Meaningful choices at every level, but not too many of them (typically no more than 2 per level)
• Numbers that advance, but within reason

Intended result, at a glance:

• Basic skill/feat system can recreate most of what 7.7 characters had using BP
• Power Source system adds back in flavorful inherent powers (think System 6 races and paths, and, of course, basic stuff like "being a wizard" or "I have a huge power armor suit")
• Magic is open-ended, skill-based, risky, and costly
• But heavy magic users can spend skills and feats to get better at it. Expertise and Mastery mitigate risks. Magic item creation really helps mitigate risk and cost (in exchange for resource cost).
• Simpler characters can invest heavily in simple power sources, such as Super Strength, or I'm a Fire Elemental. Skills and Feats can be spread broadly, or deeply focused on mastering core abilities and combat style.
• Numbers should be vaguely in line with 5e monsters (if not their crazy HP totals)
• Power list should get a bit more manageable, without the responsibility of handling abilities that should be part of Power Sources.

All PCs progress according to the following tables:

To do: briefly describe partial success, advantage, disadvantage, etc.

To do: briefly describe 7.7's initiative system.

Using supernatural abilities, such as magical skills, is costly. Strain is the primary mechanic that deals with this cost.

• Each time you use your power skills to manifest a power, or use any other ability that specifically mentions Strain, you take Strain to one of your ability scores. (Different caster types have different options)
• Over-simplifying it: if it cost 5 mana in 7.5, it'll cost 5 Strain in 7.8.
• Strain doesn't do anything in-game until it meets or exceeds the ability score in question.
• Once it does that, it imposes disadvantage on all checks involving that ability score. Furthermore, once so strained, an ability score cannot be further strained until it recovers.
• Strain recovers at a rate of 1 per ability score per long rest.

It should be quite possible, in difficult times, for practitioners to run out of available Strain. Thus, every option of Practitioner should offer at least one "lifeline", an open option for more power, in place of Strain, should they choose to exercise it. Of course, there is always a Price.

Rank 1

• You can learn magical skills such as Fire, Shadow, and Force. These are learned the same way other skills are learned, including rules for Expertise and Mastery.
• You can craft magic items, which are less costly and risky to use in everyday situations, but cost resources. Rules TBD
• Gain the benefits of a subtype:
  • Warlock: You gain the service of a minion, as well as an additional one per X levels.
  • Wizard: gain X bonus power skills for free, as well as Y free magic items, to represent the fruits of your training. You are someone's apprentice, and must obey certain rules.
  • Sorcerer: when you spend a Skill to learn a new power, you don't learn an entire power skill, but rather a single power. However, you immediately become an Expert in that power, and your Strain costs are halved (minimum 0). When you gain Mastery, your Strain costs are halved again.
  • Druid: tbd
  • Priest: tbd

Rank 2

• You can't directly invest in rank 2 practitioner, but you can learn advanced specializations, such as the below:
  • Summoner: you can summon multiple minions, they become stronger, etc.
  • Bladebound: you can junction minions to your weapons and armor.
  • etc...

Rank 1

• You can wield oversize weapons (scale +1). Improvised weapons (such as dumpsters) are still at a disadvantage to hit, but inflict double damage. Purpose-built oversize weapons (like Buster Swords) are the answer to removing disadvantage.
• Failing a Strength check means partial success, and partial success means success.
• You can use Strength checks to open locks.
• For 1 Strain to Strength, you can inflict damage similar to a battering ram.

Rank 2

• Scale 2 weapons
• Automatically pass all Strength checks that you can beat with a roll of 29 or lower.
• For 1 Strain to Strength, you can inflict damage similar to a medium-sized car crashing at 40 mph. You can effortless inflict damage like a battering ram.

Rank 1

• You have a special suit of power armor (magic, artifice, tech, whatever).
  • It is AR 4 heavy armor, but behaves as medium for the purposes of disadvantage.
  • It has two free armor enhancements of rank 1, which you can swap out for any others after a long rest.
• While wearing the armor, you gain +1 to Str, Dex, and Con

Rank 2

• Your armor improves to AR 5, and hinders as light armor.
• It now has three armor enhancement slots of rank 2.
• It also has a super-enhancement slot. Options include:
  • Jump jets: recharge 33%, jump up to 12 squares with rockets.
  • Shield generator: as the Barrier power, in some fashion
  • etc...
• While wearing the armor, you gain +2 to Str, Dex, and Con

Rank 3

• Blee

Rank 1

• Requires base race of Dragon (as System 7 dragon, i.e. rage, breath weapon, occasional transformation)
• +2 to Str, Con, and Cha
• Certain amount of free breath weapon and transformation

Rank 2

• Moar numbers
• Moar abilities