I would say do whichever one you want. Make sure to check out the modding tutorials and the quick modding-hacking answers thread if you need any further help. Just make sure that you start small, over ambition can be a double edged sword.
Whatever pleases you the most; you don't have to make mods based on what the community would like to see.
It's all in vain anyways so just do what makes you happy I suppose.
But if you do really want advice; do something that's not a CS sequel/prequel/spinoff/Alt Character Story/Challenge Mod, because all of it has been done before and nobody cares if one of the 1000 users with the name "quote" in their tag makes another one to add to the pile.
It's up to you really. I'm doing a CS retelling as my first mod, and thats had plenty of challenges, so just dont get too ambitious. Also, feel free to join the discord. We're there to help. (kinda)