The ideas are, in my opinion, should only be up to you. No matter how much you search for ideas, in most cases, you're gonna end up rejecting and instead implementing your own.
And if you plan to bring a Fortnite-ish gamemode in SA-MP, sorry to break it to you, but it won't grow you a community, because the game is already there, and it is certainly better and much more detailed than SA-MP, so normally, people would download and play the game with more features, perks and details than the stale old SA-MP.
For instance: people have made made PUBG gamemodes from scratch and they only ended up wasting their time because their ideas were fully off from the original game.
The DayZ gamemode was successful because the way SA-MP works and DayZ works were combined, e.g being able to talk and having a not-so-crowded forums (as well as form clans in a way where it's easy to get recognized because DayZ servers were distributed in small/med communities) and not only that, but there were small original and creative ideas that were added to each gamemodes, which led it to success.
TL
R,
I definitely support your plan of adding small ideas to your Fornite-ish gamemode, but mostly, the ideas are on you and up to you. It's just that in most cases people don't buy it and it won't be a sureball for a prolific or rapid growing community.