I suggest blending indoor spaces seamlessly into the big world of GTA San Andreas, drawing inspiration from a previous mod called Open San Andreas (https://youtu.be/Xy2yD5eHy40). This new idea could be really helpful for the open-source community, helping us reach a level similar to what we see in GTA IV and GTA V. Those games manage to mix the outside and inside of buildings really well.
Something similar was tried in SA:MP. They changed the texture of a building near L.S Beach Side when you use SA:MP, so you can go inside.
To start, we could work on CJ's safe houses. After that, we could move to places he often visits like Grove Street and the Police Stations. Later on, we could expand to cover other important areas too.