How about having some extra regeneration for staked SPS? This would give some nice added utility and promote SPS staking.
For example:

• 5 extra battles/day for 4M+ SPS staked (max possible)
• 4 extra battles/day for 1M+ SPS staked
• 3 extra battles/day for 250k+ SPS staked
• 2 extra battles/day for 60k+ SPS staked
• 1 extra battles/day for 15k+ SPS staked