With the digimon, I was thinking there's a few things that could be done for diversity.
1) Alter the stat system.
Instead of two stats and hp/mp, switch the stats to Strength, Spirit, Defense, Resistance. Also add some modifier stats such as Critical, Block, and Avoid.
Spirit: Raises damage with certain specials (Examples: Angewomon's Holy Arrow, Agumon's Pepper Breath)
Resistance: Defense against Spi-Based moves
Block: % chance for your digimon to block a hit, reducing the damage by 1/4.
Critical: % chance of blows inflicting 1.2x normal damage.
Avoid: % chance for your digimon to completely evade an incoming attack.
2) Add "Attack Types" that categorize each digimon based on it's strengths and gains.
Strong Attacker: +10 hp, +2 Mp, +4 Str, +2 Def, + 3 Spirit, + 1 Res a level. Base block: 8%, Base Critical: 6%, Base Avoid: 6% (Ex. of this type would be Agumon)
Defender: +16 hp, +3 Mp, +2 Str, +4 Def, +2 Spi, +4 Res a level. Base Block: 12%, Base Critical: 4%,Base Avoid: 3% (example here would be Hagurumon)
Swift Warrior: +8 hp, +5 Mp, +4 str, +2 Def, +3 Spi, +2 Res a level. Base Block: 0%, Base Critical: 11%, Base Avoid: 10% (example of this would be falcomon)
Spiritist: +5 hp, +10 mp, +2 str, +2 Def, +5 Spi, +3 Res a level. Base block: 4%, Base Critical: 2%, Base Avoid: 13%
3) Add "Stat Initializers". These chips could be found off enemies (only found off enemies) and are fed to the digimon to add very small amounts to the Block, Avoid, and Critical statistics.
4) Cap out Block/Avoid/Critical @ 20%. If they go higher, with the way byond works, there's a high chance people'd block/crit/avoid too much xD
5) Since it's boosting; make digivolutions above rookie drain a small amount of MP for every second you remain digivolved. 1% a second maybe. To counter-balance this; mp could be regenerated while in rookie form and not battling enemies.