What I'd like to see done with rare techs is to add a lot more of them, but limit how many of each tier an empire can get per game. Which technologies you'll eventually get are determined at the start of the game, so you can't just ignore them until you get the ones you want. These can then offer very interesting options that completely flip around the way you plan planets or ship designs. For example, technologies that unlocked new ship sections letting you use them for new roles, say a destroyer hanger or corvette artillery which also come with the option of installing new computer strategies. Another might be a special building that lets you refine food into alloys or a mineral synthesizing plant similar to the hydroponics farm. Some of them also have 'regular' technologies you get later that upgrade them, so they remain relevant in the end game. Current rare technologies that are required for certain things like psionic theory either stop being rare entirely, or don't follow the new rare tech rules. Several current technologies could potentially be moved over to these rare techs instead, such as disrupters (and maybe get a little buff).I'd say it's more an issue with the number of actually rare techs being very low. Most purple techs are just boring stat upgrades that happen to have 4 times less chance to be drawn than regular techs of that tier unless you have a spark of genius, curator or area expertise scientist.
If rare techs had really low chance to appear but also had really powerful effects they could actually alter the way you play. Think of stuff like giving habitats +1 district, slaves grow 50% quicker, or making robots assemble twice as fast (not for machine empires obv). That's game changing.
Technological ascendancy's effect could instead be, 'your empire gets one more rare technology per tier'.