Nefermandias
Hero
I'd say leave Healing Word as it is. Back in 4e, Healing Surges were the primary daily resource a party had to manage. Now in 5e, it's spell slots. I don't see why you would want to impose an additional cost to Healing Word.I saw a more generic form of this idea from @Garthanos and I thought it might make an interesting house rule for healing word.
Healing Word: (remove base healing). The target may spend 1 hitdice. For every level above 1st this spell is case, the target may spend an additional hitdice.
A nice way to make the spell a little more costly as it pulls from your "reserves" and puts an implicit limit on it. This contrasts with cure wounds which is "endless" magic but slower, which seems a nice differentiator between the two.
Just think like that: a 1st level Cleric in 4e could use Healing Word twice per encounter. A 1st level Cleric in 5e can use it twice in a day and at the opportunity cost of casting something more useful like Bless or Spiritual Weapon.