I am in the process of redoing a basement bathroom in which I need to relocate all the drain locations to one extent or another. I would like to reuse the old 4" toilet drain if possible so that all the repaired concrete would get covered by tile. Where the new toilet is going to go is in the SW corner, if I can tie into the old line I would need to run from the new toilet 4' to the NE to the old one and then tie in with a wye/tee (more on that in a bit). From there I can see that the old line ran to the NW. Everything from here is an educated guess based on the direction I can see the drain lines initially heading ( I have the floor removed in an 18" square around them) The old pipe runs to the NW, as I said, 3' or so then ties in with a wye/tee with the sinks 2" line. After tying together they head to the SW 5-6' and then wye/tee with another 2" drain coming from the upstairs kitchen sink which also has a vent. They then head to the West 12 foot the main stack which also has a vent. If you follow all that it would make a big U-turn NE,NW,SW, then W.

Would that matter as long as it has fall?

That first wye/tee that I mentioned is where I would like to tie in a 2" shower drain is that at all possible at that point?

I'm Ok with the big return bend but if you're planing on a 4 X 2" combination wye and eighth bend, ( it's not a wye/tee) to pick up the shower you're going to have to run a vent for it. Code mandates that EVERY trapped fixture shall be vented. It also does not allow a major fixture, such as your toilet, to discharge past a unvented minor one, such as your shower. You may revent the shower vent back into a existing dry vent if you wish.
Good luck, Tom