Let's say I have built a profitable (primary) corporation, I could then do an IPO and put it on the stock market. If it becomes overvalued or if I see better value elsewhere, I could divest and move the proceeds into a new corporation. This corporation could be started from scratch or acquired. I could then try to maximise return on equity through this new vehicle.
I know that the IPO/merger/divest etc. thing can be done to some extent using subsidiaries (Subsidiary DLC) and believe me I have done this many times. But I find it strange and limiting that I'm forced into having this connection with a particular corporation just because it's the one I started with. I view corporations as a vehicle to generate returns on capital, and so the key is really the player and the player's capital, not the corporation itself.
Just my 2 cents

EDIT: I should also add that in addition to the above that it would add a lot of fun if subsidiaries could be managed by the player as well and not necessarily by an external "AI" CEO. Basically the player should be able to be CEO of several companies at a time (like in real life), with the possibility of hiring an external "AI" CEO when appropriate.