A president removed from office does not have to be barred from holding future office. The Constitution provides that removal from office may be accompanied by disqualification “to hold and enjoy any office of honor, trust, or profit under the United States,” but Senate practice is to vote on disqualification separately.
There is some thought that the Democrats might bargain with Republicans to vote for removal of Trump from the presidency, but not to disqualify him from running again. The operating theory here is that Trump would be so weakened that he would no longer present a realistic danger of being elected. This would almost certainly be a fatal error for the Democrats, as Trump will be even more popular than ever after he is memorialized as a martyr of the Deep State. The so-called “Trump base” would turn out in record numbers, and the Trump haters would perhaps be so demoralized by his phoenix-like capacity for resurrection and their disgust with the Democratic leadership for letting Trump off the hook that they might well sit out the election.
You have to take account in this scenario of the fact that Mike Pence would now be president. He would have to choose between his own ambition and his loyalty to President Trump when deciding whether to run for election as president or reelection as vice president in 2020. But let’s face it, if Trump ran against Pence for the nomination, the recently ascendant Pence would have almost no chance of victory, as Republican support for Pence is largely proportional to his support for Trump.