Need a history lesson for this one: for decades, bus service in Downtown Tampa was split between two transit centers. The first and largest, Northern Terminal, was underneath the I-275 viaduct and primary served all routes heading north or east. Southern Terminal was the other transportation hub, primarily servicing Westbound routes. ST is currently known as Dick Greco Plaza and serves as a transfer point between the TECO Line Streetcar system and the In-Town Trolley. Union Station and Tampa Greyhound also had their own facilities independent of the other terminals, which have not changed in a very long time.
In the mid-1980s, Marion Street Transitway was constructed to give buses a path to travel between the two transit centers, as a means to make transfers more convenient for passengers connecting with routes serving different transit centers. The situation was still pretty convoluted, so in the late 1990s, plans began to construct Marion Transit Center as a replacement for the NT/ST arrangement. Once MTC opened in 2001, Marion Transitway was repurposed toward primarily being a facility for express routes to pick up/drop off passengers downtown, with former ST routes using the corridor to pass through downtown.
The 2000s saw a voter-approved mandate for High Speed Rail connecting Tampa with Orlando and Miami. As part of the project, rail would shift away from Union Station to a new downtown intermodal center located adjacent to MTC and I-275, which would end up being the all things transit facility you describe (
see the downtown study, page 9 for the basic location). That facility was planned out, but eventually killed when the FHSR project was cancelled. Florida DOT, HART and the State of Florida still own most of the land that would have been used for the intermodal center.
At some point, if current ambitions remain as they are now, All Aboard Florida will build a rail line between Orlando and Tampa, likely in the middle of I-4 in place of the former Florida High Speed Rail right of way. The terminus for that line would likely be at the former proposed intermodal center, which would functionally replace Union Station. Megabus already picks up at MTC, sharing bus bays with cross-bay commuter service from neighboring transit agency PSTA on the MetroRapid island. Redcoach prefers Tampa International Airport and the University of South Florida for their pickup/dropoff points as a sort of long distance student shuttle service for USF students. Greyhound remains a huge question mark.
In other words, transit is slowly consolidating into a single site, but it's taking a really, really long time to pull it off.