Porsche and Penske could be a thing with Porsche pulling out of GTLM. I honestly think there is no way Porsche stays away from IMSA for more than two to three years. Porsche's footprint is huge in Atlanta, it wouldn't be feasible for the investment they have put into the US market.
MSR I could see that happening, but I do not see Shank balancing IndyCar and Prototype racing. Now, imagine Herta and Marco getting involved considering they still have Mike Curb and company behind them. The guy who owns US Concrete loves Herta, Marco, and loves racing. Maybe shift RHR and Marco over the project to rejuvinate those two while giving Kirkwood more experience behind the wheel.
It would be a shame to see Caddy leave again, they have been killing it and I'd like to see them stay long term and if GTD is that expensive well I'd be calling to combine GTLM and GTD at this point. If its anything to make more affordable for privateers to participate. Maybe closer to GTE/GT3 regs? Sports car racing is hurting.
Porsche/Penske is a thing that might actually happen from what I've heard/read. Obviously they had the RS Spyder program back in the ALMS days together, and then the one year of Grand-Am DP that was forgettable.
I did see again today that there's at least one GTD team angling for the Acuras and I don't know who else that could be besides MSR. You could assume it would be at expense of the GTD NSX program.
Not sure about Herta, because if anything it sounds like he'd like to move his team up with Hyundai, but if Andretti could do it it might not be a bad landing spot for RHR, maybe Marco too. Depends on what the extent of the relationship is though. With Penske the driver lineup has been an equal input of Penske (Helio/JPM/Pagenaud) and HPD (Cameron/Taylor/Rossi) preference.
FWIW I don't think GM will leave the top rank entirely, because they're private efforts with some factory support it really boils down to which teams can find funding and which can't. WTR have that with Konica Minolta and JDC have that with Mustang Sampling and a pro-am car in the Banana Boat. Can Whelen help keep afloat the #31, we'll see.
I don't really know what to say about GTE/GT3 at this point besides they probably can't co-exist much longer. Pro and Pro-Am GT3 sounds good but the implementation is probably more difficult than that (how much more power can they extract, etc.). But you can't just have Corvette go back to running around by themselves.
I don't think there are inherently a bunch of issues with sports car racing because IMSA was going to have its largest full-season grid before the pandemic hit, ELMS has been doing well and WEC at least had direction for once but it will require a lot of evaluation and adjustment. Sports car racing has traditionally been very cyclical to begin with.