Absolutely the 1500m world record, or rather, the mile world record (which I believe Hocker has a better shot of breaking).
The World Athletics Ultimate Championships are a new thing. Basically World Athletics' answer to Grand Slam Track. It will certainly do better than GST since it's sanctioned by World Athletics but it's not proven like the World Championships or the Olympic Games. Athletes will show up because of the prize money, but it doesn't have the established prestige of the WC or OG.
Hocker has had a historic year. He's now the fifth man in history to win a global title in both the 1500m and 5000m after Nurmi, El G, Lagat, and Jakob. He has historic strength also, running 7:23 for 3K indoors and sub 13 numerous times for 5K.
Meanwhile, Hocker doesn't yet have a WR or WB to his name. He nearly got the short track 3000m WR but was nipped at the line by Fisher. His big rivals; Jakob, Kerr, Fisher, and even Nuguse all have had WRs and/or WBs to their name.
2026 is a unique year where there is no (regular) WC or OG, and GST will also not be coming back. So athletes will therefore be prioritizing record attempts on the Diamond League circuit.
If I were Hocker, I'd be putting all of my energy into going after a WR next year. While Hocker has strength, he won't break the outdoor 3000m, 2 mile, or 5000m world records (and probably wouldn't break the 2000m WR either). He could go after the indoor 3000m or 2 mile records at Millrose next year if he would like, but I could imagine him dropping down to the Wanamaker Mile in 2026. As great as Hocker is, he likes to have a sharp peak and I don't imagine him threatening Jakob's incredible 1500m/mile indoor world records. Also, Hocker's not a front-runner or time trialer; he runs his best performances when drafting behind and out-kicking someone.
As for outdoors, he could go after the 1500m or mile world records. Given that only 4 men in history have broken 3:27 (including one that was a confirmed doper and 2 that were likely dopers - the only one of our era to do it thus far was Jakob), it's a tall order to break 3:26. Since he ran 3:27.65 after 2 rounds in the OG (Jakob was "technically" everyone's pacer in that race), I believe he, when peaked, definitely has the talent to break 3:27, but not sure about 3:26. I'm not even sure Jakob can break 3:26, especially with his recurring achilles injuries.
As crazy as it sounds, I think the most likely outdoor WR for Hocker to break is the mile world record. Yes, he has never been in contention in the Bowerman Mile, always running 3:47-3:48 when the winner was 3:43-3:45. But if someone like Nuguse or Jakob is in the race, with the wavelights set to 3:43.13, and Hocker is in his prime, WC/OG-level form, then he can stick with the "WR-challenger" and outkick them like Noah Ngeny almost did to El G in 1999 (assuming weather conditions are good, of course, and they are on WR pace thru 1500m).