I recommend Brahms Handel Variations
Very difficult piece! I don't know if the OP could manage right after ABRSM 8.With such a broad field of possibilities, you could really do anything. Like I don't really know where you want to go after Grade 8. What do you want to play? What have you not done as much of?
I don’t really do abrsm or anything like that, I have no idea what abrsm 8 is except that my level is probably a bit higher? So take this with MANY grains of salt.Chopin has a lot of romantic repertoire which I think might be up to your level, I really like op 48 no 1 and ballade 1 and 2 but Idk if they are around abrsm grade 8. Rn, I’m learning some duets, they can be really fun if you have a friend to play with. It can also be new and refreshing if you haven’t played many. But duets are not for everyone, and might not help your technique as much. Since you mentioned abrsm, maybe go for whatever was above it? What was it? Dip abrsm? I’m not very knowledgeable on things like abrsm.
Hmm any kind of piece is fine, but I think I would prefer romantic pieces
Here are some bigger romantic pieces that I think are really effective considering their difficulty:Schubert Moments MusicauxChopin Mazurkas Op. 24Chopin Mazurkas Op. 50Schumann PapillonsSchumann KinderszenenSchumann WaldszenenFauré Romances Sans ParolesBorodin Petite SuiteFauré Nocturne Op. 97Play what you like. I can't help you choose between any pieces once you know of them.