So, let me check some things here...... You want:
1. Press a button to activate 'sprint' mode.
1a. Play specific animation
1b. Add motion blur
2. Disable attack/block keys while sprinting.
Yes
1. Would like the the button configurable in-game? Via a menu/spell/other key press? Or, maybe an ini file? Or, hard coded? (don't really like that option)
2. Do you want to limit the duration of sprint?
3. Do you want a fatigue drain during sprint? If so, did you have any rate in mind? Or, simply disable fatigue regeneration while sprinting?
4. What about hotkeys? should those still work while sprinting?
1. yes, via a menu.
2. you can sprint until your fatigue bar hits 0
3. yes, sprinting drain 4/5 more fatigue than running, also disable fatigue regeneration.
4. yes, you can still use the hotkeys while sprinting