We need more synergies between cards, most of it comes from the summoners, cards almost have none.
Synergies provides combinations - Combos between cards making them more useful together creating lots of strong, interesting or fun teams of cards avoiding repetitions like in case of players have the cards the best options for every mode is almost the same, more variation is good for the game, is fun and more important will make the perfect bots not play so perfect anymore since a team with not the best cards with synergy could have a good chance against the top tier cards for the ruleset that the bot will play.
It also will help the market, people will want cards that are not so good as long as they have synergy with at least 6 others cards to allow a full team, this avoid this cards to drop in price and make the market follow them, not a game changer but is something.
Some cards that currently have some type of synergy: The ''knock out'' ability with Stun ability , there is very few cards for this combo more if you are lucking for an optimize one with the stun card having higher speed than the knock out one also it get complete cancel for the immunity ability making it not so viable, others: Weapons Training, Inspire , Resurrection, Martir and that is all that we currently have almost half of it was recently introduced with the new soul bound rewards, Switfness and Strengthen are not exactly synergies as they give the bonus to all cards not specifics ones.
Some that are very commune in others games like Magic the Gathering or Hearthstone, are: giving bonus to all that cards of certain type like +1/+1 to all elf cards, may be a good idea having monsters with type and have some monsters that can buff this cards.
A few examples for splinterlands existing abilities:
Poison stack: A monster that give that ability that poison can stack( maybe each poison after the first one just increases damage by 1 for the poison) right now poison is an anti- synergy 2 or more cards with this ability are not very effective and there is a summoner that give this ability to everyone.
Poison guaranteed: A monster that increases the chance of your units poison the enemies by 50% making your units not missing when trying to poison, there should be options to avoid RNG if you don't want like a card like this one.
Poison Through immunity: A monster that make your monster with poison ability been able to poison monsters with immunity because immunity negate complete any combo based in debuffs, there should be a way to counter it.
Recharge Synergy: Any synergy for recharge ability please , they need desperately, could be true strike or higher accuracy, go through divine shield and armor, +1 damage.
Earthquake: A monster that can cause earthquake more than one Earthquake can damage units, this one provides synergy with flying units and others ability could be designed around this one like immune to earthquakes could be interesting since you can lose flying and sometimes you don't want to pick flying because of Snare will remove all your evasion.
Similar like this poison abilities could be for others abilities like the Stun ability.