Players are able to game just about any game mode as long as all troops are available. See delves, you can easily rush to level 500 without a single loss, even with the imposed color restrictions. It only gets challenging once you are seriously limited in forming a team plays well together.
I’d like to see a deck building game mode. A sequence of battles against progressively tougher opponents. Possibly represented as some randomly generated road you follow from start to end, with some steps containing mutually exclusive choices. Not all steps should be battles, some could provide positive/negative buffs or other advantages/disadvantages that persist throughout the road. You start out with a small set of loaner troops to build your team with, e.g. 8 random common troops, no traits, no bonuses. You get additional troops to use as you progress, e.g. as battle rewards. You can reconfigure your team any time you want, subject to the troops in your road pool.
1.) [Fight easy battle, get random rare troop added to pool]
2.) [Fight moderate battle, get random epic troop added to pool] or [Get Hellhound added to pool]
3.) [All future opponents get Water Link] or [All future opponents get Earth Link] or [All future opponents get Air Link]
4.) [Select troop in pool, transform it into a random one of same rarity] or [Fight moderate battle, pick between War Sphinx and Winter Wolf to get added to pool]
5.) [Fight easy battle, opponent troops will be two levels higher in all future fights] or [Fight tough battle, your troops will be one level higher in all future fights]
6.) [Fight tough battle, pick two of four random epic troops to get added to the pool] or [Fight tough battle, pick one of two random legendary troops to get added to the pool]
7.) [Select troop in pool, get additional copy added to pool] or [Select troop in pool, get all traits]
8.) [Fight easy battle, all troops used get removed from pool]
9.) [Fight tough battle, all future opponents take 50% more skull damage] or [Fight tough battle, all future opponents take 50% more spell damage] or [Fight murderous battle, all future troop picks are one rarity level higher].
10.) [For the next battle your whole team starts with full mana] or [For the next three battles your last troop starts with full mana].
X.) [Collect the Phat Loot. Cheer.]
Lots of ways to expand on this. Could possibly be a roguelike mode, lose a fight and you have to restart, pick difficulty for shorter/longer roads. Could possibly also introduce some new currency that allows you to permanently improve your future odds, e.g. by adding more starting troop picks, adding slight stats boosts, adding buffs for the first few fights, adding limited number of extra options to pick from (like 1x [Pass], move on to the next step).
Sort of like Delve, just with a lot more randomness involved, requiring you to improvise to survive.