Well, the two samples I made for testing were fairly shoddy things easily done by other means. But it probably isn't worth picking up just as a new function without some good use right off the bat for it.
Main thing you can do with the Dynamic UU that you cannot do with other means is to allow only a single Civilization to have access to the change, and having some variance in who that will be. Like you could have the Ride of the Nine Kings grant Mounted Mercenaries as a UU. That way if someone other than the Hippus manages to get the Wonder, they can ALSO have Mounted mercs.
Or you could have The Necronomicon grant a UU version of the Drown which has +1
, to grant incentive to control the Holy City as the OO. The Order could do the same by providing a UU version of the Royal Guard which is not flagged as "Will Abandon Civ" if you possess the Code of Junil. Mines of Gal'Dur could provide an "Armored Cavalry" UU of the Horseman which has enhanced Defensive Strength, and is allowed to recieve Defensive Bonuses.
First person to research Strength of Will could get a UU Djinn with an extra point of Affinity toward each Magic Type. First person to get Infernal Pact could gain a UU version of Skeletons which are Unitcombat_Melee, and can thus gain Promotions.
It also allows for some "sneaky tricks," like allowing the Barbarian Civilization to have a Unique Technology which is gained automatically when they enter the T2 Stage that grants a UU version of the Goblins or Warriors that is much stronger, so that the Barbarian continue to field units which pose a threat later into the game.
It would also be enjoyable for the Mimics to utilize in their Py-post Combat Win function, so that they completely change into the new unit instead of just picking up a promotion. But that would cause them to lose the Py-Post Combat since that is by Unit, not Promotion. However, you could use the feature slightly more creatively and have the Py-post combat assign the defeated unit as a UU version of the Champion, update all existing units to that new class, and then re-assign the Champion back to Mimic. That means that all existing Mimics (in whatever form they currently are) will change into the newly defeated creature type, and you will have to build a new Mimic and send him out hunting to change that form again.
EDIT: Example of Mimics:
I build 5 Mimics, then I send them out and one kills a Bear. I now own 5 Bears (which are actually Mimics). Those bears kill a few Goblins, and nothing happens (except the Goblin dies and the Bear who killed him gains some XP). I now build a new Mimic and send him out to kill a Boarding Party. That Mimic and the 5 Bears (who are really Mimics) are all suddenly changed to Boarding Parties. I rampage across the world with them for a while capturing ships and having a blast, but I am down to my last one and tired of playing with the sea, so I build a new Mimic. By some fluke of luck, the Mimic manages to kill Acheron. Now that Mimic and my Boarding Party (who is actually a Mimic) both become Acheron! Fortunately I picked up Mobility 1 & 2 while that other Mimic was a Bear way-back-when, so he is able to move about. Many turns later, I see the Mithril Golem has been created, so I churn out some Mimics to try and get an army of Mithril Golems. Advancing on the Beast with my stack of Mimics I am ready to dominate the world when suddenly the tricky Luchuirp player tosses a scout at my Stack-O-Mimic and suddenly I am the proud owner of a stack of 20 Scouts (1 of which had been a Bear, Boarding Party and Acheron in his life...). The Mithril Golem laughs heartily and crushes them all before I am able to get a backup troop to the scene.
