Figure Chistu will try to expand in the Hinterlands next. Secure more territory, just build-up everything. Figure he'll train the sibbies by using them offensively. Looking at the latest map (Tamar Rising page 72), there are unclaimed worlds toward the Alyina Mercantile League. Or he could try to take out Malthus Confederation. If he wants a stronger opponent, the Tamar Pact could be a target.
The sibko-grab seems to have been basically just a start but an extremely dangerous, if not even impossible, stunt to repeat. Not because it is unusual for the Clans, more like it was so humiliating to the Rasalhague Dominion.
The Horses seem too established and powerful to be a viable target, except perhaps small raids for material at best. Maybe. Currently re-reading Tamar Rising, need to bring myself up to speed again.