I've just started DMing a D&D 5e game and we're playing fairly fast and loose with the rules since my party's in it for the roleplaying more than the specifics.
One of my players is a half-orc fighter wielding a two-handed warhammer and a shield. She asked if she could (in a single round) stow her shield, take a strike with her warhammer (with both hands), then get her shield out again. From what I can tell in RAW this wouldn't be allowed since it would take essentially three actions – one to switch to the warhammer, one to attack, and one to switch back to the shield, but I allowed it at the time since I didn't see any reason not to as a house rule. We've been treating switching weapons as a bonus action anyway so in our eyes it's just an extra bonus action per turn and it does make it impossible for her to do anything else on a turn when she chooses combat.
I don't particularly care if I'm right or wrong on the rules here (since we're fine with house rules) but I am wondering if allowing this will cause me trouble later on. Are there downsides I haven't predicted to her having her shield always accessible in every round of combat going forward? It bumps her AC but as the lone tank in a 2-player L4 party they're fairly underpowered in general and this seemed like a decent balance, but that may be my naivete talking.
The biggest "issue" I predicted was just that it would give her basically a permanent bump to her AC. As addressed though, since she's the sole tank and I've still managed to hit her at least a few times in most encounters I don't see this as something that can't be dealt with by boosting monsters a little to match, if needed.
I'm mostly concerned this might affect things in later levels that I haven't researched yet; this is my first time playing D&D myself rather than watching others and I'm far from fluent.
Best Answer
Ultimately, this is all your call as DM and how/if you want to accommodate your campaign development to make any changes.
From a rules perspective, players get a free action (PHB pp190) that allows them to do things like draw/sheathe weapon, pick up a dropped item, etc.
Shields further require an action to don/doff (PHB pp146). Given that they add +2 to AC, that is a big boost and possibly why they require the action in order to gain such a bonus.
The use of two-handed weapons can also be seen as a balance issue. You are choosing bigger attacks over more defense. If you opt to let someone with a two handed weapon also get the benefit of the shield, you are greatly increasing the capabilities of that character.
There is a downside to allowing them to switch between their 2-handed weapon and shield at will: any out-of-turn actions (like Opportunity Attack reactions) will be without their primary weapon. They'll be using unarmed strikes or improvised weapon which would deal either only 1d4 if using the shield to attack (no proficiency bonus), or 1 damage if using unarmed strike.
Letting them don/doff the shield without penalty during combat basically gives them an extra 2 actions if you go purely by the rules AND they still get to get the damage benefits of the two-handed weapon. If they also have the Great Weapon fighting style and/or feat, then the balance becomes even worse.
Many of the combat/feat/fighting styles lean towards either being a DPR tank or a damage soaking tank. Letting someone get both could unbalance, but you as the DM are generating the combat scenarios and can accommodate for anything you give (or don't give) your players.
I also wouldn't get too hung about that character "being the only tank." Having a tank isn't required, just like having a healer isn't required.
Potential Solutions
Provide a means of access to magic protection (better armor, magical armor, ring/cloak of protection).
Give them access to Shield of Faith (via feat Magic Initiate), but still maintain the concentration mechanic. This will give them the +2 they're looking for, but at a risk.