In order for me to get braces 16 years ago I had 4 teeth removed behind my canine teeth. I had braces on for 3 years due to moving and college changes, horrible. Anyways, after 3 years I couldn't take them anymore so I had my braces removed two months early. I know, I know but I was just done. Anyways, I had a small, small gap left. I thought no big deal, you can't see them. I wore my retainer at bed for years. Anyways, to make a long story short my teeth all started to shift and now the gap is gone but my teeth aren't perfectly straight. And for reference I had my braces removed at 23 as an adult so I shouldn't of had that much movement unlike children that get braces. I would suggest some sort of dental implant or bridge.
My son had a sick tooth removed and they built him a gap retainer that would keep that space open for him.