These are some questions/ideas for thresh and jungle interaction.

I will put the questions first so they can be answered. Other stuff will be there but I don't necessarily expect an answer it would just be cool.

Question 1
What jungle mobs drop souls?

Question 2
Could a support/mid Thresh wander into the jungle to gather souls from their jungler clearing (say helping to damage wolves/blue for early souls).

Question 3
In relation to question 1, would it be expected for jungle Thresh to have less souls than lane Thresh?

Question 4
Not exactly jungle related but what is the range on Thresh's passive (how close must he be to units that die in order for souls to drop)?

I hope you could give at least short answers to those

Okay now for my take on ways to make Thresh's jungle better.

Passive
I would hope that all large jungle creeps drop souls with a chance of smaller ones dropping them as well

Q Ability
This is what I see as his main damage source. His E is okay but the CD is long and the base damage is not good. Something like the passive deals X minimum damage to monsters would help his early clear so that he can take attack speed runes and get that nice auto attack single target clear.

W Ability
I do not see much potential for jungling here. This skill has too much utility to make it stronger.

E Ability
This skill has potential for quicker clears the biggest problem I see is the low base damage and long cooldown. I think reducing utility vs minions&monsters and trading it for damage would be a nice approach. Something like minions&monsters are not knocked back or slowed but take 50% increased damage. After all you really want the sweep for champions.

R Ability
No jungler should need this to jungle (besides Udyr who it is a must).

I like Thresh and I hope to see him tweaked so he is better in more roles than just support. Thank you for reading this if you do.

1: Large ones always will. Small ones do sometimes (RNG).

2: Yes.

3: Yes (not always, of course, but thresh's jungle clear is awfulbad, whereas he doesn't have to actually do anything in lane aside from be there and pick up the souls)

4: Pretty generous (I sorta want to say Nami-ult-esque)

To be fair, his Q passive does have a minimum damage per attack - it's just that minimum damage is however many souls you've picked up.

anyhoo, the dude is perfectly fine as a support or maybe-top pick. There's not a lot of reason to change that (not like we don't already have Nautilus if you want a similar-but-better jungle experience)