I think it's a great idea to give people a formal supporting role in your child's life.
In our case, they are Godparents and all are members of our church. But having seen how valuable they are, I'm sure we would want to have a similar set-up even if we weren't religious.
We have the same set of Godparents for both our children. They are a married couple with slightly older children ("the Godsisters", who take their role very seriously!) and an older lady who is about the same age as our parents. We see them a lot and they have a genuine role in supporting us and being special friends/mentors to our daughters.
They have done copious amounts of babysitting, provided lots of imaginative and thoughtful presents for the girls, been called out in the middle of the night to help with medical crises and generally been there when we needed them. And it has deepened our friendship into something quite special.
I would say that it is very valuable to ask people who you actually see a lot, because it makes the role so much more meaningful for the children. DD1 (now 5) is so close to all her Godparents and loves having them around.
Having an older person has also been great, because all the grandparents live a long way away, so it has been a good way of bridging the generation gap and valuing the role of an older generation in the girls' lives.
Go for it!