I know they're already doing the whole "cast in a higher slot" thing, and that's a great start, but would it be beneficial (and I'm thinking it would be) to combine more spells, balled up into a couple of since there's no real term for what I'm thinking, let's call them "iconic class spells" [tm].
Here's what I'm thinking...
Clerics: Cure Wounds & Bless.
Cure Wounds: For each spell level heals d8 +caster level hp of damage. Cast in a 3rd level slot: Also removes blindness, deafness, paralysis or disease. Cast at a 5th level slot: Also can be used to repair ability damage and energy drain ["Restoration"]. Cast at a 7th tier slot: Auto heals to full for an individual or can be cast en masse [let's say] 5' radius (or 10' diameter, if you prefer) per caster level (which, assuming 5th level spells at 9th level, is a 45'radius or 90' diameter).
Bless: For each spell level offers +1 to hit and saving throws to 1 person per caster level. Cast as a 3rd level slot: Also grants advantage regardless of other conditions/circumstances. Cast as a 5th level slot: Also can be cast to apply to all allies within 5' radius per caster level.
Druids: Control Plants and Control Animals [or summoning or something]
Control Plants: Cast as 1st level slot: Entangle. Cast at 3rd level slot: Also acts as Plant Growth. Cast at 5th level: i dunno. Something uber-super plant-y.
Control Animals: Cast as 1st level: Speak with Animals. Cast at a 2nd level slot: also Charms Animal. Cast at a 3rd level slot: Also acts as Animal summoning/Nature's Ally who is then charmed/obedient. Cast at a 5th level slot: Dominate Animal...etc...
Wizards: Magic Missile and "Mystic Barrier"
Magic Missile: For each spell level used produces 1 bolt, does 6 hp of damage each, automatically hits. At 3rd level slot: 3 bolts, damage 6 hp each, or 30' diameter burst of energy, 18 hp to all within area of effect At 5th level slot: 5 bolts, damage 6 hp each, or 50' conical blast, 30 hp to all within range. etc...
Magic Barrier: Cast 1st level slot: Shield. Cast 3rd level slot: Acts as shield but can be cast as 15' diameter globe. Cast 5th level slot: Globe of Invulnerability (blocking physical and magic attacks)...etc...
Illusionists: Color Spray and Phantasmal Force
Color Spray: 1st level normal. 3rd level slot: increase effect somehow...5th level slot: Prismatic Spray...etc...
Phan Force: normal audio-visual...3rd level slot: major image, visual/auditory/olfactory/infrared, maybe extends without concentration...5th level slot: shadow/pseudo-real image...less or no concentration necessary...etc...
Does this make sense/work...or have I jumped the spellbook?