If we really want to use a broad brush and define the founder of "aikido", then it would have to be Sokaku Takeda. There are no others.
I dunno..... since the whole concept of "becoming one with your opponent" (and related quotes) goes back into antiquity, as does the "intent", use of the hara in internal strength, internal strength itself, kokyu power, and so on, why say it all started with Takeda. As I posted years ago on this forum, all of this stuff is nothing but another small part of a much bigger picture that has gone on in Asia for a couple of thousand years. I.e., first you see only the art around you and think that's the origin of everything; then you begin to see the history of a generation or two; then you begin to understand the bigger picture.
Even narrowing down the scope of the picture to the provincial level of Ueshiba and Takeda, it would be presumptive to credit Takeda with "aiki", given how widely known the topic was ("Kuzushi", "use the opponent's force" in the jin sense, etc.). I.e., Takeda didn't come up with "aiki" on his own..... more probably, our information of all that happened is just incomplete and we're assuming too many times that what little history we know represents all there was. All we can do is get our brief glimpses of the players, understand that our perspective is limited, and then go practice.