The central mystery is this:
1) We get you elected by bot propaganda
2) We let you build Trump towers in Russia and Eastern Bloc countries.
3) We don't release our Kompromat on you.
It's that fucking simple.
Yeah, all this seems plausible but it still leaves tons of questions. Let's examine those points as possible motivations for Trump:
Engineering the election: How did Russia pitch this idea that they could tilt the election? Why would Trump and his associates believe the Russians had the capability to do this (it's not like they're well-versed in clandestine active measures.) Mercer and Bannon were into micro-targeting and weaponizing social media bubbles but it wasn't a proven strategy at that point. While Manafort might have had some experience with Belarus and Ukraine operations, even people within the intelligence community hadn't fully grasped the implications of cyberwarfare against democratic processes. This also presumes Trump desired the presidency in the first place (ego, revenge on Obama, legacy) but if he's truly a puppet, how do we know his runners didn't push him in this direction?
Real estate deals as payment: If Trump wanted huge payments for his involvement, why tie his rewards to the Russian state... he's an East Coast real estate mogul, why not have all the money flow into his US-based assets through corporate shell games? The Putin regime could seize or squeeze any Trump businesses in Russia any time they like, similar to the way they took everything from Bill Browder with the stroke of a pen and a few busted heads. And since he'd be deeply compromised in this scenario, there is nothing he could have done about it... Let's remember that the oligarchs want more than anything to get their capital OUT of Russia where it isn't vulnerable to thieves like themselves or a public uprising. Trump would be an utter idiot if that's what he accepted as payment. Not saying he isn't but we really need more evidence here.
Coercion through blackmail: We're talking about the 'grab em by the pussy' guy, who bragged about being able to shoot someone on Fifth Avenue and get elected anyway. I'm not sure how character assassination would faze him so maybe the kompromat involves legal jeopardy, like money laundering. circumventing sanctions or vices more heinous than what we know already. For all the hype about the Moscow Ritz Carlton tape, it really doesn't qualify as a major piece of kompromat for this particular guy. There is also the possibility the blackmail targets his family members and he's motivated by the desire to protect them... again, we don't know at this point.
Some people act as if Mueller is severely lagging behind things that are 'obvious' from all the (excellent) investigative journalism but every revelation throws up a ton of new scenarios and lines of inquiry for the investigators.