They've flirted with this idea already a few seasons ago when they tried out an LTM where you could pick up various ultimates off the ground. I can't remember if you could use them instead of your own or in addition to your own. And honestly, this is more or less a feature of Wildcard with all the nonsense you can pick up out of a replicator. Wall running. Self-revive. The infamous Dash. The list goes on. My own experience so far has shown me a lot of added complexity without a lot of added benefit. Not to mention that some of these abilities (specifically Dash) interact poorly with my legend's existing kit. So not exactly a seamless experience.
I understand that people want to mix things up and have new experiences, but in my opinion there is already far too much bleed between different legends. Some of it is because newer legends are just rehashes of older legends. Some of it is because they put some legends' ults in packages anyone can pick up (mobile jump tower). And still some more comes, as I said, in the form of Wildcard insanity. I know you're bored. I'm a little bored too. But I'm not really seeing this as the way forward. Unfortunately for me, the developer seems to think you're on to something, so don't expect it overnight, but over the next few seasons? I'd bet on it in some form or other.