My best guesses would be:
GWR 111 'The Great Bear' - following on from other experimental steam locomotives (W1, Turbomotive)
New Tool Caledonian Single - there seems to be an appetite for pre-grouping Scottish locos (Rail/Bach CR 828, Rapido HR Jones Goods) and Hornby are protective of locos that they have made in the past.
S&DR Locomotion No.1 - this is less likely but I expect a new loco for the Rocket range and Locomotion makes sense with the upcoming bicentenary of the S&DR, although maybe they'll wait for the 2024 announcements.
Another Peckett/Industrial steam loco - maybe a Manning Wardle, the 0-4-0 the SECR used would make sense.
Another Ruston/Sentinel/Industrial IC loco - Not sure what to expect here, but there’s sure to be something.
A fireless or battery loco - bit more of a wildcard but I think Hornby will want to keep things interesting.
009 Narrow Gauge - this is more wishlisting but Hornby have shown willingness to enter new scales, so you never know. 00n3 is more likely as it uses TT gauge track.