I just got put back into league 4 as I don't yet have 40k SPS. However having been 5 leagues in, I agree the game could use some tweaks yet to change things up. I use the same 3 towers and 3 spells on probably 90% of the maps regardless of rules in play.
In that spirit and to encourage the play/usage of more of the game's tower's and spells, what if you took things a bit further though and added 4 new possible game rules that deny the use of each level of rarity of game asset. A game rule each for No Commons, No Rares, No Epics and No Legendaries. To really spice it up allow up to 2 of those new rules to pop out of the possible 3 game rules for the day maybe.
I'm not sure, but that may create situations where a map and rule combo are not possible to complete or maybe not possible until certain leagues when the tower or spell stats get just right. Imo that is fine, the challenge will be who can score most with what is allowed.