Allow a hefty normal capacity (for various reason often to do with gun/tactic combinations to experiment with)
But maybe give some well define bonus's for keeping a light load :- to make it a goal worth pursuing
Faster movement/maneuverbility, quicker reactions, lower exhaustion rate, fitting thu smaller holes, being able to go prone, etc...
Sufficient availability of required tools/resources to 'grab' for specialized situation would be required (and sometimes is a 'dead give away' that such situations are approaching)
Player would have option of when to discard for advantage or to hoard inventory/tools