Not gonna lie, Soldier X + Hunch + Daggermaw + Sewer Rat does sound like a cool Master Splinter build. Otherwise we could just cut the Gordian Knot and just give him every Potency 1 sample.
If rocking those 3 as bases its something like these options? Enhanced Strength + Clairvoyance + Enhanced Senses is the assumption currently, going to hard limit this to 8 cost so we never do more than one Fast Pass due to money. This would leave roughly 6 directions we could take with this imo for trying to strengthen it within that cost range?
Sewer Rat + base (6 total, no boosting needed) = (2x all) Enhanced Senses, Transformation, (1x all) Clairvoyance, Enhanced Strength, Fitness, Regeneration, Self-Enhancement. Building up the Enhanced Senses and seeing if Self-Enhancement pushes it further. If going Fast Pass probably just grab Steadfast or Multiplex/Millions.
Adamant + base (6 total, no boosting needed) = (2x all) Transformation, Enhanced Strength, Regeneration, (1x all) Enhanced Senses, Clairvoyance, Fitness. Building the Enhanced Strength and seeing if ABO helps it defensively. If going Fast Pass, probably just Steadfast?
Frostbite + base (6 total, no boosting needed) = (2x all) Enhanced Strength, (1x all) Elemental Manipulation, Regeneration, Enhanced Senses, Clairvoyance, Transformation, Fitness. Building the Enhanced Strength and Invuln, likely goes Fast Pass to get Hydrocity + 1 (Multiplex or Millions) unless going for Steadfast.
Golem + base (8 total, needs Fast Pass) = (2x all) Enhanced Strength, (1x all) Elemental Manipulation, Regeneration, Enhanced Senses, Clairvoyance, Transformation, Fitness. Building the Enhanced Strength and some Invuln, lacking Hydrocity sucks but capping at 8 here so.
Memoria + base (7 total, needs +1, realistically just Fast Pass) = (2x all) Clairvoyance, (1x all) Teleportation, Enhanced Senses, Enhanced Strength, Transformation, Regeneration, Fitness. Building the Clairvoyance and hoping battle senses + Teleportation + Enhanced strength allows for a combat teleporter. Frankly unless Fast Pass costs extra its probably better to add on a 1 cost as well and I'd go Millions for trying to get more combat Teleporter Memoria synergy?
Fast Track + Steadfast + base (8 total, needs Fast Pass) = (1x all) Enhanced Senses, Enhanced Strength, Self-Enhancement, Ally Enhancement, Enhanced Speed, Invulnerability, Transformation, Fitness, Regeneration, Clairvoyance. This is basically trying to see if we can enhance this build off of going for every enhance synergy boost on top of the baseline Senses/Strength, where Steadfast is required just to fix the Fast Track issue.