Here's a prey that's simple but challenging: Smug Slime
A slime girl with a short shirt saying "drink me" and her face is smug as if she's saying "c'mon. I dare you. Bitch. Just see what happens"
She's worth a ton, but upon digestion the pred loses digestion (or capacity if you can work around glitches like over capacity or no capacity
The idea is to present a challenge to well established fields (that the right word?)
This leads me to another idea for consideration.

Here's some ideas I have on the prey deck. I mention these because it touches on the previous idea a little but also might expand on future idea possibilities
Some prey only show up at certain parts of the game, or at least are more likely to.
This is to keep troublesome prey (like Smug Slime) from screwing players over early on.
Some prey could show up only early on to make catching them more challenging.
A special store upgrade might make certain prey show up early on to help build your field quicker.
Then there could be super rare cards that only have a chance to show up on every tenth turn.

*edit* last idea here: all I'm gonna say for now is BOSS CARDS. Not gonna get into how they work now. Just gonna leave it as food for thought

Hope I didn't ramble too much. If my ideas are unclear, don't work with what you're trying to do, or just make a mess out of the code, feel free to let me know. I want to be helpful

There are a lot of preds the eat the prey orally not many that do it anally or with unbirth. I would like a change or and option to  change  what type of vore a card does.

Now, this would probably take a good amount of effort (and it might be impossible since I don't make games), but what if there was a card that turned a prey card into a pred card (which would be added to your deck), with corresponding effects (Mufi and Sachiho being snowballers like the Mobi variants, princess not able to be removed, etc)

Another thing I think would be nice is if there wasn't caps (for digestive power and capacity) in infinite mode like when you get that one power up in campaign, or maybe instead a system (in infinite) where earning points would allow you to buy power ups.

maybe just a minor option menu to toggle any death related dialog off? Just like...incase someone doesn't want to see that kinda dialog 

more unbirth preds

Idea for a pred: one that is able to level up an unlimited number of times, something like 3/3 for 3 energy. Starts very weak but scales up. I suspect this would be a weaker scaling strategy than something like Nexxi or Mobi, given the number of copies you can have is inherently limited and it would require a big energy investment compared to those, so to make it more viable, each level could also raise their maximum capacity by the same amount, allowing them to potentially scale bigger than any other pred if you have enough to stack up. Would have some synergy with Plum and Seraph, potentially.

On the repeated proposition of pred-on-pred, while having a pred who can eat the others is understandably a no-go, having one that can be eaten by others could be valid. Basically a pred who can be "leveled up" into any other pred, transferring their stats over. Could make for a fun strat paired with a pred that removes rules from prey- basically letting that pred indirectly gain the effects from beneficial prey like Mufi, Sachiho and Angel. An option could be the ability to play them on a seated pred for a small buff, so the card isn't completely dead if you draw them late.

Alternatively, there could be some way to "pick them up" from their seat and feed them to another seated pred, but that would probably be finicky to implement.

A switch prey could also be possible, one that gives more points for each other prey in the belly (to represent them "eating" the other occupants).