Sorcerer
Sorcerers have the full spellcasting progression; any changes to the Spellcasting feature will have a big impact on the class.
Like bards, sorcerers are have a limitation on the number of spells they can choose from, which is a major restriction on the class.
The sorcerer does not get many metamagic choices. When you create a new metamagic option, be sure that it is useful enough that a sorcerer could justify using one of his or her precious choices on it.
Sorcery points and Flexible Casting were intentionally designed so that a sorcerer who does nothing but convert spell slots to sorcery points in order to cast higher-level spells does so at the cost of overall output. Be cautious when altering this balance.
Warlock
Warlocks have a unique spellcasting method, and they rely on being able to cast a smaller number of spells more frequently. Remember that a warlock automatically increases the spell slot level of spells he or she casts, meaning that even lower-level spells gain potency when cast by a warlock.
The warlock spell list was carefully cultivated to avoid including spells that might become annoying if cast too often at the table. If you want to grant a warlock access to a new spell, but are concerned that its frequent casting could be disruptive to the game, consider creating an eldritch invocation that enables the use of the same magic on a more limited basis (by requiring a rest between uses, for instance).
Warlocks derive a lot of their combat potency from the eldritch blast cantrip, and already have a lot of invocations to choose from to increase that reliance. Be wary of creating new invocations that make eldritch blast even more powerful.
