Manbearcat
Legend
Running with @Manbearcat's example, here's a possibility adapted shamelessly from the Dungeon World druid (but also informed by how Burning Wheel handles failed Faith and Spirit Binding checks):
When you call upon the support of your patron in desperate or dangerous circumstances, roll 2d6+ CHA bonus:
* On 10+, choose two:
• Your supplication is granted.
• You patron demands no payment from you.
• Your patron takes nothing in payment from your friends and allies.
* On 7+, choose one of the above.
* On 6-, your patron is fed up with the supplications of mortals, and inflicts some disaster upon you and/or your friends and allies. The GM will tell you what bad thing happens.
If you were adapting it to 5e, the numbers would have to change: it's hard to maintain the exact DW probability spread, but my tentative suggestion would be d20 + CHA bonus, with full success on 16+, partial success on 8 to 15, and faiure on a natural 1 and on any modified result below 8.
That's pretty much where I was going with my follow-up post if we felt like we addressed all the issues!
Above you have:
1) Player choice in the possible manifestation of their character's patron's power.
One might say "but the dangerous and/or desperate rider puts the deployment under the GM's purview." To that, one simply points to fact that GMs are meant to constantly fill the PC's lives with adventure, think dangerously, and the snowballing nature of the resolution mechanics means that each situation is going to be fraught with danger (and often desperation)!
2) A propensity for aid, but a propensity for demanding something in return (under GM purview following the agenda/principles/rules).
3) The potential for the patron to become offended and the fallout to be severe, but interesting (GM purview following the agenda/principles/rules).
Player has specified authority over their thematic portfolio.
Thematic focus amps up when things go badly.
The (transparent) rules have their say.
The GM has constrained, guided authority (which mitigates cognitive workload).
That all seems like a win to me?