Welcome - it's good to hear from you especially as you mention wildlife issues.
We have a wildlife pond and, as far as I know, we are not thinking of anything bigger. It supports amphibians, and visiting birds and insects are catered for too. The margins are planted with native British plants (though some aliens arrive, and goldfish, also unasked for, are occasionally 'donated').
The pond has a group of volunteers to maintain it, and you are welcome to join us; look in the photos to see some of us at work and if you want to be on the regular Volunteers newsletter, let me know.
Jo Corke wildlife convenor.
The park Is about 5 hectares in size and sloping, so a proposal for a big lake would be an ambitious project. You clearly know the park, and if you have seen it the summer you will know how popular it is and the pressures on the available space.
You ask how to proceed. The park is run by Bristol City Council. They encourage local communities to form friends groups to liaise with BCC. I think it appropriate to refer your idea to our FOSAP chair, Steve Perry by email and cc you.