People have posted to this forum several times looking for ideas for breakthroughs or wanting to run their ideas by the Ars population at large for feedback. I think that a collection of cool breakthroughs (especially lesser breakthroughs) that fit comfortably within the rule set would be a great resource for folks to refer to when these sort of threads pop up and it would be a boon to the community at large serving as a springboard for lots of stories and ideas.
Naturally the creation of such a list is probably impossible because everyone's ideas of what fits comfortably within the rules differs. Furthermore when folks post ideas that differ from your conception of what the rules should be it isn't particularly easy or even desirable to refrain from discussing how you think the proposed breakthroughs are problematic.
Yet even realizing that the task is quixotic I'm eager to get to work on it. When you're offering criticism to ideas that others have posted please try to make your response as rules and game specific as possible. The sort of criticisms that I expect to see are :
"I believe that hermetic magic can already accomplish this so a breakthrough is unnecessary, look at the perdo stuff guidelines on pXX" or
"This breakthrough messes with the limit of the soul so I don't like it", or
"This breakthrough would destroy the reason to have specific sorts of stories such as X in the game and therefore I think that it would do more harm then good"
So I'll start
Target pair at the same level as target part (this would help sponting imensely)
An muto vim enchantment that can target other enchantments in a familiar bond and thereby allow bond enchantments to target something other than the familiar or the magus (preferably under the control of the familiar). This is arguably possible under the rules as they stand.
A breakthrough that reduces the pawns of vis necessary to study an art from 1 per five levels to 1 per eight or ten levels. This is a boring breakthrough but you might introduce it later on in a game as the result of NPC research so as to keep vis study a viable alternative for experts.
Creation of rego, muto and creo imagonem enchantments to radically increase the quality of a text by creating a multi-media experience. This is also arguably already possible within the guidelines as they presently sit.
a major one
Research to make magical potencies teachable, researchable and compatible with hermetic theory. In other words give characters an opportunity to give themselves minor magical potency virtues by undergoing specific ritual, laboratory and research activities (like a non-mysterious mystery initiation, you know, involving stories).
A hermetic level one (inspired by the new book):
Combine the arts of mentem and corpus into a single art (like animal) by integrating the understanding of the physical makeup of the ten senses with the understanding of the body.