Here is how to make Tryndamere's ultimate more interesting to play with and against:

Tryndamere ultimate makes him immune to death for as long as an enemy champion was within 300 range in the past 2 seconds, rather than just a flat 5 seconds.

WHY?

So it has more risk/reward, so his team can work with him, and so the only counterplay isn't "outrun him" for 5 seconds.

So to put the change into perspective, here are some example use cases:

Tryndamere spins into a messy team fight where everyone is clumped up, nobody bothers to spread out, tryndamere is cc'd for almost an eternity but he isn't dying, he starts smacking things, now people spread out but probably not before he kills somebody.

Tryndamere spins into a team fight, bursted down to 1 hit point and then knocked away by lee sin, now he has to close the 1200 unit distance within 2 seconds or be vulnerable to death.

Tryndamere walks into a team fight, gets blown to 1 hit point, sees that the squishy carry isn't far from him so he spins out of the clump and into the carry, the carry flashes and tryndamere gets snared, then people just stay away from him for 2 seconds and he gets killed by an auto attack.

Tryndamere walks into a team fight, people ignore him completely and he obliterates everyone.

Tryndamere walks into a team fight at baron, people burst him and use all their cc on him but don't try to actually spread out because they really want baron and baron pit is hard to move around in, so he just stands there and takes it for 3 or 4 seconds, they may or may not actually get baron, then he obliterates half of them as they try to funnel through the narrow entrance to leave.

Tryndamere walks into a team fight in mid lane, the fight ensues and tryndamere gets put down to 1 hit point, gragas throws an ultimate that perfectly knocks tryndamere both away from his team and away from the enemy team, then tryndamere dies to maoki a few moments later.

Tryndamere walks into a team fight, gets snared by morgana and people just move out of his 300 range danger area and then he dies to an auto attack.

Tryndamere walks into a team fight, gets cc'd, people move out of his 300 range but tryndamere's ally, diana, doesn't want tryndamere to end up dead before he can fight so she uses her pull to put some people back into range long enough for tryndamere to get back in the action.

Tryndamere walks into a team fight, gets knocked away by jayce after trying to kill a carry, is about to become vulnerable to death, but then lee sin pulls a sweet ward hop + kick that puts jayce within range of tryndamere and he lives for a little longer.