I think Mew as a starter is a terrible idea. Lack of version exclusives is silly too. Pokemon was split up into two versions for the sole purpose of linking up to trade and battle and such. I know you're not saying the trade function would be removed, but it's still odd to think of a game without version exclusives to where you actually need to, you know, find a trade partner. It's not like it's even a hard thing to do when you have this forum and Wi-Fi. But hey, I'm just a cynic that's against all these "ultimate Pokemon game" ideas. They won't happen and don't need to happen. And that first file of the game would pretty much be useless once you've transferred your ish to the second game. It just...doesn't work well, honestly.
And let's not forget that they'd be unbalanced. Kanto and Johto on the first cart, Hoenn, Sinnoh, and Unova on the second. The latter three regions are definitely bigger than the first two, with more to do (thinking of their games). So not only do you have 2 regions vs 3 (and no, Sevii Islands do not count as a region), the other 3 are bigger than the other two. It just doesn't add up. So they'd end up nuking the second game with its big clump of regions, making them less enjoyable like GSC's terrible Kanto, while the first version would flourish with two regions, which we know the DS and 3DS are capable of doing, as seen in HGSS. I would rather have one good game with one developed region than two carts that span all five, but are severely unbalanced.