My ideas for techchasing Fox (all assuming either that the grab isn't perfectly synched, or your opponent would be able to mash out before you could start a Wobble)
Situation A: Near the ledge, facing towards the ledge (but not close enough to handoff)
- D-throw
- Move slightly towards them (close enough to grab without getting hit by shine)
- Prepare to react for tech in place/no tech
- If camera bounces, they missed the tech, if camera stays still, tech in place
- Both tech rolls are easy to react to; away is limited by distance to ledge, and towards you is shortened because you're already standing in the middle of the roll distance (you can cover both with normal dash -> grab)
- Keep going till you get a Wobble/handoff
- With Sopo you can fish for d-throw -> no DI -> reverse d-smash, or cover multiple tech options with d-smash
Situation B: "Medium distance" (about half to two-thirds of Battlefield) from the ledge, facing away from the ledge
- B-throw
- Immediately wavedash at a distance that will put you in grab range
- Look for tech in place/no tech
- Covering tech rolls is pretty much identical situation A, but you have to be careful not to wavedash too far
Situation C: Near the ledge (too close for b-throw), but facing away from the ledge
- D-throw
- Immediately turn around and wavedash to the other side of Fox
- Tech roll towards the ledge will be an unpopular option, since it's easy to react to and puts them in a dangerous situation
- Tech roll away becomes much easier to cover since the distance is effectively shortened
- Still look for tech in place/no tech
- Once you get the regrab, refer to Situations A or B
Situation D: Not near any ledge (or medium distance + facing the ledge)
- This will never occur on smaller stages (Yoshi's, Battlefield, FoD)
- If Nana is synched, you can go for a mid-stage handoff, d-throw -> u-smash, or f-throw/b-throw -> ice block
- The ice block makes it easier to cover tech in place/no tech
- However, it doesn't help with tech roll away, which is probably your opponent's safest option; if they reach the end of the stage it will usually push them onto the ledge before you can get there
- D-throw techchases are possible, but it's difficult to cover every option
- On Stadium and FD, I would prefer u-throw combos in most cases